doc: clarify PR commits in contributor guidelines

[edkerk]

Applying squash or rebase merge on PRs from devel to master seems to result in the next PR between those branches still indicating the same commits as difference (+ any additional commits). To prevent this, in the contributor guidelines should be states that PRs from devel to master should always be a 'normal' merge commit. PRs from feature-branches to devel would preferably be rebase merges, to keep the git tree tidy. Note that there is no way to set this behaviour as default.

To do:

• make changes in contributor guidelines