Update README.md

Author: julienrf

README.md

@@ -10,3 +10,9 @@ repository. The GitHub workflow `.github/workflows/generate-docs.yaml`
 periodically executes the script `.github/scripts/generate-docs.scala`,
 which generates the website content and pushes it to the repository
 [scala/docs.scala-lang](https://github.com/scala/docs.scala-lang).
+
+Note that the state of the proposals in the “design” stage is defined by
+their [labels](https://github.com/scala/improvement-proposals/labels),
+whereas the state of the proposals in the “implementation” stage is defined
+by their YAML frontmatter (e.g.
+[SIP-42](https://github.com/scala/improvement-proposals/blob/583b458e3a7c0d2310e1a71a3812d73f7f6efebc/content/binary-integer-literals.md?plain=1#L1-L7)).