Update file engineers.md

While working on MR gitlab-org/gitlab!130823 (merged), I have noticed that our docs are a little vague when it comes to creating an MR of the latest stable branch and to improve dev efficiency while performing this task, I decided to clarify some details.

To some this MR could be redundant as I am stating possibly obvious steps, nevertheless, dealing with such a crucial task like backporting that does not happen very often, I think clear and simple steps outlined will be appreciated by anyone who needs to follow them.

I also mentioned, that adding an MR for master branch is needed. I think that even thought this is so logical, but in the stress of the backporting, it can be potentially forgotten, chance why I added it to make sure it gets done.