Contribution guidelines do not cover how to PR fixes for docs

[jankapunkt]

The documentation does require to start PRs from development. However, the readthedocs.io always points to master and fixes for stable docs should therefore be made by pointing to master and starting from master.

Otherwise we might face the uncomfortable situation of

• either merging unstable code from development into master
• or have to wait for the fix to be released until development is stable and mergeable

The contribution guidelines should therefore have their own section on how to contribute to the docs.

See #220 (comment)

[jorenvandeweyer]

Can we set the default branch to merge to development instead of master?

[jankapunkt]

@jorenvandeweyer done