Add instructions for merging branches

@silabs-robin, it occurred to me that the summary of the merging operation you added to core-v-verif pr 1125 is very usefull institutional knowledge that is worth preserving. This PR creates modifies a presentation created by Steve Richmond, adding a new slide (no. 12) titled "How to merge" that attempted to capture what you wrote in the core-v-verif discussion. I say "attempt" because I did not fully follow the last two bullet points.

Please review this and suggest better language for the slide.