Add RPC doc blurb into contrib readme

Author: karelbilek

contrib/README.md

@@ -7,3 +7,6 @@ Contains the script `github-merge.py` for merging github pull requests securely
 
 ### [Verify-Commits](/contrib/verify-commits) ###
 Tool to verify that every merge commit was signed by a developer using the above `github-merge.py` script.
+
+### [RPC documentation generation](/contrib/doc-gen) ###
+Script for generating RPC documentation files from a running Bitcoin Core instance.