How to Add a File Type (decoder or encoder)

Read code_guidelines.txt.

1) add FF_<filetype> constant to corona.h
2) add proper function to Open.h or Save.h
3) call function in CorOpenImageFromFile or CorSaveImageToFile in Corona.cpp
4) add Open<filetype>.cpp or Save<filetype>.cpp with implementation of
   your function...  I recommend basing your decoder on OpenPCX or
   OpenTGA.
5) update doc/doxygen/mainpage.doxy
6) update src/Corona.cpp (CorGetSupportedReadFormats and
   CorGetSupportedWriteFormats)
