Doc how to get integer value of mapped enum
Doc how to get integer value of mapped enum

Added some documentation on how to get the integer value of an enum that is mapped to integers.

From https://github.com/brainspec/enumerize/issues/131
shaundunne/enumerize,brainspec/enumerize,quarkstudio/enumerize,tranvictor/enumerize,tanraya/enumerize,victor95pc/enumerize,monfresh/enumerize,hungrything/enumerize,maurogeorge/enumerize