Update README.md

Author: jimenezjose

README.md

@@ -43,6 +43,9 @@ $ cd build
 $ java MazeGUI 
 ```
 
+### Known error:
+If an error is thrown in the `make` command in step 4, execute `mkdir build` then continue from step 4 again.
+
 <!---
 ## Getting started with the [STM32](https://github.com/jimenezjose/STM32)