Please consider adding instructions on how to build RE2 under Windows

[Maxim Masiutin]

Please see https://github.com/google/re2/pull/309

and https://raw.githubusercontent.com/maximmasiutin/re2/513ffc1d153b7d7ccba32a414be6ce18e8edd50f/README-WINDOWS.md

or https://github.com/maximmasiutin/re2/blob/513ffc1d153b7d7ccba32a414be6ce18e8edd50f/README-WINDOWS.md

Consider adding this text to RE2 repository to make people's life easier.

Also, if somebody may additionally provide instructions on how to build it for Windows with CMake, it will also be good.

There were no instructions on how to build RE2 for Windows.

While on Linux it is quite straightforward to build, and you have provided the instructions in the README file, it was not that obvious for a general user on how to build RE2 for Windows using native Windows tools like Microsoft Visual Studio.

I propose the small instruction. The file is named in a similar fashion as notes on how to build OpenSSL on various platforms, e.g. https://github.com/openssl/openssl/