[boost] [1.56.0] Release notes

Hello all, If you haven't written up your release notes yet, now is a good time. You can create a pull request on github by clicking on 'edit' on this page while logged in: https://github.com/boostorg/website/blob/master/feed/history/boost_1_56_0.qb... Alternatively, you can clone the site locally and do it using the normal git procedure, or you can email me and I'll add it for you. thanks, Daniel