docs: update README.md

Author: NGPixel

README.md

@@ -26,6 +26,7 @@
 - [Docker Dev Environment](docker/README.md)
 - [Continuous Integration](#continuous-integration)
 - [Database & Assets](#database--assets)
+- [Old Datatracker Branches](https://github.com/ietf-tools/old-datatracker-branches)
 
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