add/update documentation for plugins

Signed-off-by: Steve Kriss <steve@heptio.com>

docs/plugins.md

@@ -0,0 +1,35 @@
+# Plugins
+
+Heptio Ark has a plugin architecture that allows users to add their own custom functionality to Ark backups & restores 
+without having to modify/recompile the core Ark binary. To add custom functionality, users simply create their own binary 
+containing an implementation of one of Ark's plugin kinds (described below), plus a small amount of boilerplate code to 
+expose the plugin implementation to Ark. This binary is added to a container image that serves as an init container for 
+the Ark server pod and copies the binary into a shared emptyDir volume for the Ark server to access. 
+
+A fully-functional [sample plugin repository][1] is provided to serve as a convenient starting point for plugin authors.
+
+## Plugin Kinds
+
+Ark currently supports the following kinds of plugins:
+
+- **Object Store** - persists and retrieves backups, backup logs and restore logs
+- **Block Store** - creates volume snapshots (during backup) and restores volumes from snapshots (during restore)
+- **Backup Item Action** - executes arbitrary logic for individual items prior to storing them in a backup file
+- **Restore Item Action** - executes arbitrary logic for individual items prior to restoring them into a cluster
+
+## Plugin Naming
+
+Ark relies on a naming convention to identify plugins. Each plugin binary should be named `ark-<plugin-kind>-<name>`,
+where `plugin-kind` is one of `objectstore`, `blockstore`, `backupitemaction`, or `restoreitemaction`, and `name` is
+unique within the plugin kind.
+
+## Plugin Logging
+
+Ark provides a [logger][2] that can be used by plugins to log structured information to the main Ark server log or 
+per-backup/restore logs. See the [sample repository][1] for an example of how to instantiate and use the logger 
+within your plugin.
+
+
+
+[1]: https://github.com/heptio/ark-plugin-example
+[2]: https://github.com/heptio/ark/blob/master/pkg/plugin/logger.go