More CONTRIBUTING.md updates

(cherry picked from commit 0d2c627)

Author: michaelklishin

CONTRIBUTING.md

@@ -9,8 +9,8 @@ The process is fairly standard:
 
  * Present your idea to the RabbitMQ core team using [GitHub Discussions](https://github.com/rabbitmq/rabbitmq-server/discussions) or [RabbitMQ community Discord server](https://rabbitmq.com/discord)
  * Fork the repository or repositories you plan on contributing to
- * Run `gmake`
- * Create a branch with a descriptive name in the relevant repositories
+ * Run `git clean -xfffd && git clean && gmake distclean && gmake` to build all subprojects from scratch
+ * Create a branch with a descriptive name
  * Make your changes, run tests, ensure correct code formatting, commit with a [descriptive message](https://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html), push to your fork
  * Submit pull requests with an explanation what has been changed and **why**
  * Submit a filled out and signed [Contributor Agreement](https://cla.pivotal.io/) if needed (see below)
@@ -35,6 +35,7 @@ killall -9 beam.smp; killall -9 erl; killall -9 make; killall -9 epmd; killall -
 cd deps/rabbit
 
 # cleans build artifacts
+git clean -xfffd
 gmake clean; gmake distclean
 
 # builds the broker and all of its dependencies