branches.md: Add mellanox and multiproc branches

[lukego]

Register two new branches for collecting and integrating feature branches that incrementally create a couple of large new features:

• mellanox: Collect the changes for ConnectX-4 support until ready for master. See Plan for Mellanox ConnectX-4 support #706.
• multiproc: Collect the changes for multiple process parallel processing until ready for master. See Add lib.ctable module. #722.

The main intention of creating integration branches is to make it easier to break down the development into smaller independent pieces and have a place where we can fit them together.

I propose to be the subsystem maintainer for multiproc and mellanox branches. So if somebody sends a PR on these topics then I would be the upstream person merging it onto the appropriate branch. Then once the branch as a whole is in good shape I would send a PR and ask somebody else to be upstream for that.

So the overall workflow would be like:

feature -> multiproc/mellanox -> max-next/wingo-next/kbara-next -> next -> master

This allows individual feature development branches to be reviewed and accepted even before the subsystem they contribute to has landed further upstream.

Hope it is a good workflow!

[mention-bot]

By analyzing the blame information on this pull request, we identified @eugeneia and @wingo to be potential reviewers