add troubleshooting for missing Fortran compiler

Author: zdaq12

README.md

@@ -97,6 +97,11 @@ error: [Errno 2] No such file or directory: 'cmake'
 ```
 Try rerunning after installing CMake (e.g., `apt-get install cmake` or `brew install cmake`)
 
+```
+No CMAKE_Fortran_COMPILER could be found.
+```
+Try installing a Fortran compiler (e.g., `brew reinstall gcc`)
+
 #### How to debug
 ```sh
 git clone --recursive git+https://github.com/open-atmos/PyPartMC.git