Update the binaries README for 3.0-rc2 and MinGW binaries.

Also indicate that MSVS 2013 binaries are available and correct the name of
the headers archive.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75075 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775