Document Test-Driven Workflow in CONTRIBUTING.md; Document PR audit file requirement

[oberstet]

Bug Description

Same as crossbario/crossbar#2152

Regards files:

oberstet@amd-ryzen5:~/work/wamp$ find . -mindepth 2 -maxdepth 2 -name CONTRIBUTING.md
./wamp-proto/CONTRIBUTING.md
./crossbar/CONTRIBUTING.md
./cfxdb/CONTRIBUTING.md
./wamp-xbr/CONTRIBUTING.md
./zlmdb/CONTRIBUTING.md
./txaio/CONTRIBUTING.md
./autobahn-js/CONTRIBUTING.md
./autobahn-python/CONTRIBUTING.md
oberstet@amd-ryzen5:~/work/wamp$

GitHub links such files automatically - which is nice:

Checklist

• I have searched existing issues to avoid duplicates
• I have provided a minimal reproducible example
• I have included version information
• I have included error messages/logs