Some late feedback on 'Contribute to Guide' PR #109

[danielnordh]

I was too slow to comment on it before the merge but here are some tweaks.
Apart from fixed typos and some simplified language the main changes are:

• Added Slack channel and Github sections on contribute.md
• Suggest changing the nav title to 'Contribute to guide' to differentiate from higher level 'Contribute'
• Bump sub-headline styling to differentiate do's/don't

[danielnordh]

If/When this PR gets merged, please squash the commits as there it is a bit 'unclean', containing commits already merged.
Discussed this with @pavlenex and it seems to be the best way.

Reason: I branched off a PR that was already merged (unbeknownst to me). I then merged master into that branch to make sure there were no conflicts. But the previous commits still show up on this PR.

[pavlenex]

Just a note, since this PR is based of my earlier PR and not the master, it would be a good idea to squash commits when merging this PR.

[pavlenex]

Thanks for quickly addressing the navigation and for all the improvements to the page.