add list of plugins

Signed-off-by: zachaller <[email protected]>

docs/analysis/plugins.md

@@ -52,3 +52,11 @@ Argo Rollouts will download the plugin at startup only once but if the pod is de
 Argo Rollouts in HA mode can help a little with this situation because each pod will download the plugin at startup. So if a single pod gets
 deleted during a server outage, the other pods will still be able to take over because there will already be a plugin executable available to it. However,
 it is up to you to define your risk for and decide how you want to install the plugin executable.
+
+## List of Available Plugins (alphabetical order)
+
+#### Add Your Plugin Here
+  * If you have created a plugin, please submit a PR to add it to this list.
+#### [sample-rollouts-metric-plugin](https://github.com/argoproj-labs/sample-rollouts-metric-plugin)
+  * This is just a sample plugin that can be used as a starting point for creating your own plugin. 
+It is not meant to be used in production. It is based on the built-in prometheus provider.