Merge rust-bitcoin/rust-bitcoin#3476: Contributing: Add section about release versions and deprecation

10148adc31 contributing: Add section about release versions and deprecation (Steven Roose)

Pull request description:

  Picked up Steven's PR #2137 and corrected `NEXT_RELEASE` to `TBD`.

ACKs for top commit:
  tcharding:
    ACK 10148adc31
  apoelstra:
    ACK 10148adc31 successfully ran local tests
  storopoli:
    ACK 10148adc31

Tree-SHA512: 891d2ac4accf0667570c90d528b11a16d6d8e77068a1260b0974b0eeec142b6a2d21973278d7a280fa2efbc137f458584d7e0e02ccb357cc33352afa660a6307

CONTRIBUTING.md

@@ -125,6 +125,18 @@ adhere to the ideas presented in the following two blog posts:
 - [How to Write a Git Commit Message](https://cbea.ms/git-commit/)
 - [Write Better Commits, Build Better Projects](https://github.blog/2022-06-30-write-better-commits-build-better-projects/)
 
+### Deprecation and Versioning
+
+Whenever any part of your code wants to mention the version number the code will
+be released in, primarily in deprecation notices, you should use the string
+`TBD` (verbatim), so that the release preparation script can detect the
+change and the correct version number can be filled in in preparation of the
+release.
+
+```rust
+    #[deprecated(since = "TBD", note = "use `alternative_method()` instead")]
+```
+
 ### Peer review
 
 Anyone may participate in peer review which is expressed by comments in the pull