Improve links to open and active RFCs

This change updates the README to improve the links to RFCs.
We haven't been consistently using the "Proposed" and "Active" GitHub labels
and so the current links don't provide a good overview of what RFCs are still under discussion
and which ones have been accepted by the core team and are awaiting implementation.

README.md

@@ -9,9 +9,14 @@ bit of a design process and produce a consensus among the Wagtail core team.
 The "RFC" (request for comments) process is intended to provide a consistent
 and controlled path for new features to enter the CMS.
 
-Current RFCs:
-- [Active](https://github.com/wagtail/rfcs/pulls?q=is%3Apr+label%3AActive+)
-- [Proposed](https://github.com/wagtail/rfcs/pulls?q=is%3Aopen+is%3Apr+label%3AProposed)
+**Open RFCs (proposed new features):**
+- [New](https://github.com/wagtail/rfcs/pulls?q=is%3Apr+is%3Aopen+label%3A1%3ANew)
+- [Accepted](https://github.com/wagtail/rfcs/pulls?q=is%3Apr+is%3Aopen+label%3A2%3AAccepted)
+- [Review](https://github.com/wagtail/rfcs/pulls?q=is%3Apr+is%3Aopen+label%3A3%3AReview)
+- [Final Comment Period](https://github.com/wagtail/rfcs/pulls?q=is%3Apr+is%3Aopen+label%3A%224%3AFinal+Comment+Period%22)
+
+**Active RFCs (awaiting implementation):**
+- https://github.com/wagtail/rfcs/tree/main/text
 
 ## When you should follow this process