docs: add explanation

Author: antfu

README.md

@@ -1,3 +1,7 @@
 # RFCs Bot
 
 Automation of RFCs creation for [vitejs/rfcs](https://github.com/vitejs/rfcs).
+
+This bot is provided as a GitHub App exclusively for the `vitejs/rfcs` repo. Upon a new RFC PR created, the bot will create a linked Discussion Thread with auto generated link to rendered RFC.
+
+This is to workaround with GitHub Actions' [restriction on forked PR](https://docs.github.com/en/actions/security-guides/automatic-token-authentication#permissions-for-the-github_token) in a safe and elegant way.