Provide a community branch and describe it inside contributions

[de-jcup]

Situation

We have sometimes the situation that we have really good contributions, but we want to test them before finally merging to develop branch. This can be a long running situation which could become become very exhausting for the contributors.

Also it is sometimes a very slow process and could demotivate people.

Wanted

• For contributors it shall be faster and easier to get their changes merged
• For maintainers it shall be easier to check build and also to be able to make smaller
  changes afterwards before merging it - speed up things

Solution

• We introduce a community branch: https://github.com/mercedes-benz/sechub/tree/community
• We change documentation at https://github.com/mercedes-benz/sechub/blob/develop/CONTRIBUTING.md accordingly and mention that the PRs shall go to the community branch

[lorriborri]

We dropped community branch completely and modified the build action for PRs