doc: Add guix prefix for changes to reproducible builds

Author: hebasto

CONTRIBUTING.md

@@ -153,7 +153,8 @@ the pull request affects. Valid areas as:
   - `test`, `qa` or `ci` for changes to the unit tests, QA tests or CI code
   - `util` or `lib` for changes to the utils or libraries
   - `wallet` for changes to the wallet code
-  - `build` for changes to the GNU Autotools or reproducible builds
+  - `build` for changes to the GNU Autotools or MSVC builds
+  - `guix` for changes to the GUIX reproducible builds
 
 Examples: