Add contributing instructions

[aik099]

The CONTRIBUTING.md file needs to be added to answer common questions, that contributors might have, e.g.:

• which branch all PR's should be targeted to
• what coding standards project adheres
• etc.

This is the file I've created by looking at most popular repositories: https://github.com/qa-tools/qa-tools/blob/master/CONTRIBUTING.md

[peter279k]

It looks like the reference link about contributing guide is broken.

This link should be: https://github.com/qa-tools/qa-tools/blob/master/CONTRIBUTING.md.

[aik099]

@peter279k , thank you. I've updated the link. At moment of posting the issue (3 years ago) the link was working, but since them the branch mentioned in the link got deleted.