docs: add PR title and DCO rules to CONTRIBUTING.md

[treuherz]

This PR adds the commit convention to the CONTRIBUTING docs. This caught me out on #320, hopefully this helps the next person!

[beeme1mr]

Hey @treuherz, good catch. The requirement is only for the PR title as we squash PRs and use the PR title as the new commit message.

As a follow up, I'll update our GitHub action to post a more helpful message in the PR itself.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.88%. Comparing base (ce4920a) to head (68eb0ef).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #321   +/-   ##
=======================================
  Coverage   86.88%   86.88%           
=======================================
  Files          13       13           
  Lines        1372     1372           
=======================================
  Hits         1192     1192           
  Misses        156      156           
  Partials       24       24           

Flag	Coverage Δ
e2e	86.88% <ø> (ø)
unit	86.88% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[treuherz]

Thanks for the rapid reivew on this and #320, @beeme1mr