Add usage info to README

Author: yaph

README.md

@@ -1 +1,20 @@
-# README
+# GitHub Merged Pull Requests
+
+Showcase your merged pull requests on GitHub.
+
+## Action Inputs
+
+### `token` (*required*)
+
+Personal access token with `read:user` scope.
+
+* Generate a token at: https://github.com/settings/tokens
+* Add the token to the repo where you use this action: https://github.com/USER/REPO/settings/secrets/actions
+
+### `output`
+
+Path to generated file.
+
+## Example Workflow
+
+TODO