add building instruction hint for windows

Author: coco875

doc/building.md

@@ -55,6 +55,14 @@ cmake ..
 make -j$(nproc)
 ```
 
+_Note:_ on windows the last command is
+```powershell
+cmake --build . --config RelWithDebInfo
+``` 
+
+_Note:_ if nyan can't find flex add `-DFLEX_EXECUTABLE=path/to/win_flex.exe` to the cmake configure command.
+
+
 `cmake` registers the project in the [user package registry](https://cmake.org/cmake/help/latest/manual/cmake-packages.7.html#user-package-registry).
 Other projects can therefore easily find `nyan` **without** installing it
 (no `sudo make install` needed).