doc: add note to README on cross-compiling

PR-URL: https://github.com/libuv/libuv/pull/2682
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>

README.md

@@ -200,6 +200,15 @@ $ build/uv_run_tests                        # shared library build
 $ build/uv_run_tests_a                      # static library build
 ```
 
+To cross-compile with [CMake][] (unsupported but generally works):
+
+```bash
+$ cmake ../..                 \
+  -DCMAKE_SYSTEM_NAME=Windows \
+  -DCMAKE_SYSTEM_VERSION=6.1  \
+  -DCMAKE_C_COMPILER=i686-w64-mingw32-gcc
+```
+
 ### Install with Homebrew
 
 ```bash