add link to rfc-0001 in broker README (paritytech#1862)

# Description
- What does this PR do?
  - link added
- Why are these changes needed?
  - improve docs
- How were these changes implemented and what do they affect?
  - only concerns docs

substrate/frame/broker/README.md

@@ -2,7 +2,7 @@
 
 Brokerage tool for managing Polkadot Core scheduling.
 
-Properly described in RFC-0001 Agile Coretime.
+Properly described in [RFC-0001 Agile Coretime](https://github.com/polkadot-fellows/RFCs/blob/main/text/0001-agile-coretime.md).
 
 ## Implementation Specifics