docs: check out release version for macOS build

Author: mhx

README.md

@@ -557,6 +557,8 @@ $ pip3 install mistletoe
 - Build DwarFS and run its tests:
 
 ```
+$ git checkout v0.9.0
+$ git submodule update
 $ mkdir build && cd build
 $ cmake .. -GNinja -DWITH_TESTS=ON
 $ ninja