Add docs for cross compiling Mono for arm #2270
Description
One of the big advantages of adding Mono as a runtime target is the support for much easier builds on more obscure platforms, such as armel, which are approaching impossible to being buildable. Mono has support for many of these platforms, which would be awesome to have full .net core support on. My specific use case is armel, which does have an RID and has been well tested in Mono except for a known interpreter bug (mono/mono#14591). It would be awesome if does were added about how to set up a cross compiler with mono, or even a native compile using an existing mono .net framework installation.