misc: Updated CONTRIBUTING.md to discuss WIP changes

misc: Updated CONTRIBUTING.md to discuss WIP changes

Previously CONTRIBUTING.md discused private changes, but contributors
cannot submit private changes to Gerrit under the current configuration.
This has therefore been removed and replaced with an explanation of how
to submit a WIP (Work In Progress) changes to Gerrit. WIP changes are
permitted.

Change-Id: I9e1acbdaf4a9b5c433c0704ba9faf325a6b48c6a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24805
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>