contributing.md: don't use merge commit when doing release

[jkremser]

minor: warn that either squash or rebase has to be used during the release process

other option would be to ban the merge strategy for PRs in here (project's settings on GH):

or use an action for it that checks all the commits in the PR (checkAllCommitMessages)

The reason why merge commit fails the release pipeline is the fact that we use something like:
if: startsWith(github.event.head_commit.message, 'RELEASE:') (here) and that github.event.head_commit.message is actually a commit msg of the newest commit in the PR, but when using the merge commit, it will be the newest one and the msg will be different.

Yet another fix could be using github.event.commits[0].message which should be the OLDEST commit in the pr batch, but I haven't tried this.

Signed-off-by: Jirka Kremser [email protected]

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