Avoid release-notes.md conflicts #12819

From #bitcoin-core-dev:

12:05 < jnewbery> release-notes.md conflicts are really irritating. Any thoughts on how to make them stop? Perhaps have a separate release-notes file for each PR that requires them, eg release-notes-pr1234.md, and then concatenate/edit them before the release? Release notes need a lot of editing anyway, so might not add too much overhead
12:11 < sipa> jnewbery: that doesn't seem like more overhead than the to-wiki-and-back dance we do already at the end
...
13:23 < instagibbs> jnewbery, ive had to rebase a couple PRs a couple times each due to that :/
13:27 < sipa> instagibbs: easy rebases, thankfully :)
13:28 < instagibbs> muh acks!

I'm interested to hear any other suggestions for avoiding release-notes conflicts.

I think this can be fixed by adding release-notes.md merge=union to the .gitattributes file in ./doc/.

There has been some discussion here: https://botbot.me/freenode/bitcoin-core-dev/2018-03-29/?msg=98389096&page=4