Merge #14157: [0.17] doc: merge upstream release-notes from bitcoin-c…

…ore/bitcoin-devwiki

786825c doc: Update release-notes.md from upstream devwiki (fanquake)

Pull request description:

  sipa [suggested in IRC](https://botbot.me/freenode/bitcoin-core-dev/2018-09-06/?msg=104094793&page=1) that now would be a good time to merge the release notes from the [bitcoin-devwiki](https://github.com/bitcoin-core/bitcoin-devwiki).

Tree-SHA512: 6d9631e12147b4e64f5382a64cceb71b84ca21bdb3fc516248de79968f7f26e74ab5d569e3767018581bcda4d5b8c8ec103b1795929e98f7e9c4d6e95379f7de

Elements release with peg-in overhaul, peg-in segwit support for watc…

…hmen, and much more