[build2] build2 0.16.0 release candidate is available for testing

[Boris Kolpackov]

As promised, letting you know that the NEWS files for the main toolchain
components have been updated and you can see what's new and perhaps test
features that you are particularly interested in:

https://git.build2.org/cgit/build2/tree/NEWS

https://git.build2.org/cgit/bpkg/tree/NEWS

https://git.build2.org/cgit/bdep/tree/NEWS

The updated documentation referenced by some NEWS entries can be found
here:

https://build2.org/stage/