docs: Add RFC PR link to README.md (#139)

* Add RFC PR link to README.md

* Fix link formatting in README.md

Author: aladdin-add

designs/2024-baseline-support/README.md

@@ -1,6 +1,6 @@
 - Repo: eslint/eslint
 - Start Date: 2024-04-20
-- RFC PR: (leave this empty, to be filled in later)
+- RFC PR: https://github.com/eslint/rfcs/pull/119
 - Authors: [Iacovos Constantinou](https://github.com/softius)
 
 # Introduce a way to suppress violations