Update MSVC compilation instructions regarding path length on Windows

Author: zilbuz

README.md

@@ -129,6 +129,9 @@ CALL "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\bin\amd64\vcvars64.
 python x.py build
 ```
 
+If you are seeing build failure when compiling `rustc_binaryen`, make sure the path
+length of the rust folder is not longer than 22 characters.
+
 #### Specifying an ABI
 [specifying-an-abi]: #specifying-an-abi