Additional provider docs (#498)

* Modify providers shortcode

Signed-off-by: lucperkins <lucperkins@gmail.com>

* Add extra note to provider section

Signed-off-by: lucperkins <lucperkins@gmail.com>

* More info in Providers doc

Signed-off-by: lucperkins <lucperkins@gmail.com>

* Update Alibaba tag

Signed-off-by: lucperkins <lucperkins@gmail.com>

* Add resources directory back to Git

Signed-off-by: lucperkins <lucperkins@gmail.com>

website/assets/sass/style.sass

@@ -64,6 +64,10 @@ $colors: mergeColorMaps(("twitter-blue": ($twitter-blue, $white), "slack-green":
   .main
     flex: 1
 
+blockquote
+  code
+    background-color: $grey-lighter
+
 =hide
   display: none
 

website/content/docs/providers.md

@@ -36,4 +36,80 @@ mkdir providers/my-provider
 
 In that created directory, implement the [`Provider`](https://godoc.org/github.com/virtual-kubelet/virtual-kubelet/providers#Provider) interface in [Go](https://golang.org).
 
-For a basic example, see the [Virtual Kubelet CRI Provider](https://github.com/virtual-kubelet/virtual-kubelet/tree/master/providers/cri).
+> For an example implementation of the Virtual Kubelet `Provider` interface, see the [Virtual Kubelet CRI Provider](https://github.com/virtual-kubelet/virtual-kubelet/tree/master/providers/cri), especially [`cri.go`](https://github.com/virtual-kubelet/virtual-kubelet/blob/master/providers/cri/cri.go).
+
+Each Virtual Kubelet provider can be configured using its own configuration file and environment variables.
+
+You can see the list of required methods, with relevant descriptions of each method, below:
+
+```go
+// Provider contains the methods required to implement a Virtual Kubelet provider
+type Provider interface {
+    // Takes a Kubernetes Pod and deploys it within the provider
+    CreatePod(ctx context.Context, pod *v1.Pod) error
+
+    // Takes a Kubernetes Pod and updates it within the provider
+    UpdatePod(ctx context.Context, pod *v1.Pod) error
+
+    // Takes a Kubernetes Pod and deletes it from the provider
+    DeletePod(ctx context.Context, pod *v1.Pod) error
+
+    // Retrieves a pod by name from the provider (can be cached)
+    GetPod(ctx context.Context, namespace, name string) (*v1.Pod, error)
+
+    // Retrieves the logs of a container by name from the provider
+    GetContainerLogs(ctx context.Context, namespace, podName, containerName string, tail int) (string, error)
+
+    // Executes a command in a container in the pod, copying data between
+    // in/out/err and the container's stdin/stdout/stderr
+    ExecInContainer(name string, uid types.UID, container string, cmd []string, in io.Reader, out, err io.WriteCloser, tty bool, resize <-chan remotecommand.TerminalSize, timeout time.Duration) error
+
+    // Retrieves the status of a pod by name from the provider
+    GetPodStatus(ctx context.Context, namespace, name string) (*v1.PodStatus, error)
+
+    // Retrieves a list of all pods running on the provider (can be cached)
+    GetPods(context.Context) ([]*v1.Pod, error)
+
+    // Returns a resource list with the capacity constraints of the provider
+    Capacity(context.Context) v1.ResourceList
+
+    // Returns a list of conditions (Ready, OutOfDisk, etc), which is polled
+    // periodically to update the node status within Kubernetes
+    NodeConditions(context.Context) []v1.NodeCondition
+
+    // Returns a list of addresses for the node status within Kubernetes
+    NodeAddresses(context.Context) []v1.NodeAddress
+
+    // Returns NodeDaemonEndpoints for the node status within Kubernetes.
+    NodeDaemonEndpoints(context.Context) *v1.NodeDaemonEndpoints
+
+    // Returns the operating system the provider is for
+    OperatingSystem() string
+}
+```
+
+In addition to `Provider`, there's an optional [`PodMetricsProvider`](https://godoc.org/github.com/virtual-kubelet/virtual-kubelet/providers#PodMetricsProvider) interface that providers can implement to expose Kubernetes Pod stats:
+
+```go
+type PodMetricsProvider interface {
+    GetStatsSummary(context.Context) (*stats.Summary, error)
+}
+```
+
+For a Virtual Kubelet provider to be considered viable, it must support the following functionality:
+
+1. It must provide the backend plumbing necessary to support the lifecycle management of Pods, containers, and supporting resources in the Kubernetes context.
+1. It must conform to the current API provided by Virtual Kubelet (see [above](#adding))
+1. It won't have access to the [Kubernetes API server](https://kubernetes.io/docs/concepts/overview/kubernetes-api/), so it must provide a well-defined callback mechanism for fetching data like [Secrets](https://kubernetes.io/docs/concepts/configuration/secret/) and [ConfigMaps](https://kubernetes.io/docs/tutorials/configuration/).
+
+In addition to implementing the `Provider` interface in `providers/<your provider>`, you also need to add your provider to the [`providers/register`](https://github.com/virtual-kubelet/virtual-kubelet/tree/master/providers/register) directory, in `provider_<your provider>.go`. Current examples include [`provider_azure.go`](https://github.com/virtual-kubelet/virtual-kubelet/blob/master/providers/register/provider_azure.go) and [`provider_aws.go`](https://github.com/virtual-kubelet/virtual-kubelet/blob/master/providers/register/provider_aws.go), which you can use as templates.
+
+## Documentation
+
+No Virtual Kubelet provider is complete without solid documentation. We strongly recommend providing a README for your provider in its directory. The READMEs for the currently existing implementations can provide a blueprint.
+
+You'll also likely want your provider to appear in the [list of current providers](#current-providers). That list is generated from a [`provider.yaml`](https://github.com/virtual-kubelet/virtual-kubelet/blob/master/website/data/providers.yaml) file. Add a `name` field for the displayed name of the provider and the subdirectory as the `tag` field. The `name` field supports Markdown, so feel free to use bold text or a hyperlink.
+
+## Testing
+
+In order to test the provider you're developing, simply run `make test` from the root of the Virtual Kubelet directory.