[README] MSBuild 15+ is needed on Windows (#792)

This commit is inspired by the known issue of finding a compatible
MSBuild given that VS 2017 has SxS support and additionally installs
various versions of MSBuild (such as MSBuild 15 and MSBuild 4
depending on payloads installed).

People may try to build using an incompatible MSBuild version that
lacks proper C# feature support.

Update `README.md` to explicitly specify that MSBuild 15+ is required.

README.md

@@ -235,7 +235,7 @@ Unit tests are build in a separate target:
 
 ## Windows
 
-To build Xamarin.Android, run:
+To build Xamarin.Android, ensure you are using MSBuild version 15+ and run:
 
     msbuild build-tools\scripts\PrepareWindows.targets
     msbuild Xamarin.Android.sln
@@ -244,6 +244,8 @@ These are roughly the same as how `make prepare` and `make` are used on other pl
 
 _NOTE: there is not currently an equivalent of `make jenkins` or `make all-tests` on Windows._
 
+_Troubleshooting: Ensure you check your MSBuild version(`msbuild -version`) and path for the proper version of MSBuild._
+
 ## Linux build notes
 
 If you have the `binfmt_misc` module enabled with any of Mono or Wine installed and