Update README.md

Author: olarozenfeld

README.md

@@ -34,6 +34,8 @@ You must compile using C++20:
 bazel build --cxxopt=-std=c++20 //src:botc
 ```
 
+Note: on Windows, this option becomes `--cxxopt=-std:c++20`.
+
 You may run tests using:
 
 ```sh
@@ -44,4 +46,4 @@ We use the [Google C++ style guide](https://google.github.io/styleguide/cppguide
 
 ```
 cpplint src/*.h src/*.cc
-```
+```