[boost] [1.60.0] Release Notes

Hello everyone, If you haven't done so already, please can you add your entries to the release notes. Easiest way to do so is to create a pull request by clicking on the pencil icon at the top right of the source code at: https://github.com/boostorg/website/blob/master/feed/history/boost_1_60_0.qb... Or if you prefer you can email me. thanks, Daniel