chore: Remove link to currently-private Rivet RFCs

Author: AngelOnFira

README.md

@@ -57,8 +57,6 @@ _and much more coming soon!_
 1. Look for any issue that describes something that needs to be done - or, if
    you're willing to add a new feature, create a new issue with an appropriate
    description.
-    - You can follow [Rivet RFCs](https://github.com/rivet-gg/rfcs) if you want
-      to be up to date with upcoming features.
 2. Submit your pull request.
 3. Rivet team will review your changes.
 4. Don't forget to join [Rivet's Discord](https://rivet.gg/discord) to hang out