git instructions on wiki page

[henrikrudstrom]

collected all the git instructions from the comments and created a wiki page; https://github.com/qmlweb/qmlweb/wiki/Contribute-Code

@ChALkeR could you please see if this is correct?

also added some links that describes the general workflow, if anyone has a better/shorter link please suggest it.

Would be good to have a section on when to rebase and when to merge (ff-only or no-ff)
does this make sense to us: http://emmajane.github.io/rebasing-workflow/#/0/26

[ChALkeR]

Perhaps we should also create a CONTRIBUTING.md file?

[henrikrudstrom]

thats a good idea.

[ChALkeR]

GitHub "Rebase and merge" is good now, and I decided to remove the complexity with Reviewed-By etc.
Closing for now, will reopen if any issues arise.