Merge pull request #2954 from zeenix/doc-rfc-num

Document when and how a number is assigned to RFC file

Author: nikomatsakis

README.md

@@ -105,7 +105,8 @@ merged into the RFC repository as a markdown file. At that point the RFC is
 
   - Fork the RFC repo [RFC repository]
   - Copy `0000-template.md` to `text/0000-my-feature.md` (where "my-feature" is
-    descriptive. don't assign an RFC number yet).
+    descriptive). Don't assign an RFC number yet; This is going to be the PR
+    number and we'll rename the file accordingly if the RFC is accepted.
   - Fill in the RFC. Put care into the details: RFCs that do not present
     convincing motivation, demonstrate lack of understanding of the design's
     impact, or are disingenuous about the drawbacks or alternatives tend to