You want branch descriptions:
git branch --edit-description
This will open up your editor and let you attach metadata to the branch. You can extract it with:
git config branch.<branch>.description
A couple of important notes:
- This is stored locally. By definition it can't be pushed since it's stored in - .git/config. All the same it works great for this use case.
 
- If you delete the branch, the description will delete as well. 
- You can push this description into merge commits if you set - git config --global merge.branchdesc true. This means when you issue- git merge --log <branch>, it'll force the branch description into the stock merge commit message. This has a lot of uses. For example, this is how I track topic branch release notes at my employer.