Add guidelines for contributing

[szszszsz]

Add CONTRIBUTING.md with:

• code style guidelines (code reformatting)
• git flow description
• make as small consisting changes as possible within one PR (eg. do not mix translations and functional changes)
• do not mix functional changes with not own code reformat
• what issue description should contain (OS, stick firmware, app version)

[FlorianUekermann]

-Compile without warnings (on at least one recent compiler version).
-Pass llvm-format without changes (use a style config file if necessary)
-Don't break ability to compile (at least on amd64)
Use travis to enforce these and to always have binary builds.

[szszszsz]

@jans23 @MaVo159
Please check guidelines submitted in commit. Further additions should be made in issue_75-contributing. Please update in case of obvious errors in separate commits.

[FlorianUekermann]

The .md contents sound pretty good. I didn't look at more stuff. I'm a little confused why there are 3 commits that add the same file (is it the same?). I'm on mobile, so it is difficult to check what is going on. Anyway, looks good to me.

[szszszsz]

Great! You can visualize it under link
These are all same commit (technically different) on same branch - I have updated the content two times because of typos.