[boost] [config][release] Release notes for old compilers.

Hi, I added a little release notes for config, and also a note about removing support for older compilers in the future. If this concerns you, please give it a quick check and let me know if you want any changes. This release note is not a promise that future changes will or won't be made, and it can be changed if appropriate. Sorry that this is a bit late. We can still change the release notes after the beta (and final) release, so don't worry about that. thanks, Daniel