Auto merge of #29229 - SingingTree:reflect_28260_in_readme, r=alexcrichton

This PR adds a note to the end of the Windows build instructions to reflect the issues detailed in #28260, as well as a work around using older versions of gcc. I've avoided going into detail as I did not wish to bloat the README, and so that the changes are easy to yank once the issue is resolved.

Author: bors

README.md

@@ -87,6 +87,13 @@ Read ["Installing Rust"] from [The Book].
    $ ./configure
    $ make && make install
    ```
+> ***Note:*** gcc versions >= 5 currently have issues building LLVM on Windows
+> resulting in a segmentation fault when building Rust. In order to avoid this
+> it may be necessary to obtain an earlier version of gcc such as 4.9.x.
+> Installers for earlier Windows builds of gcc are available at the
+> [Mingw-Builds] project. For more information on this see issue #28260.
+
+[Mingw-Builds]: http://sourceforge.net/projects/mingw-w64/
 
 ## Building Documentation