Modify USAGE.md a bit
Modify USAGE.md a bit

(GLES2 backend is now usable, default for NPOT is to reflect then GLES backend extension)
ptitSeb/gl4es,ptitSeb/gl4es,ptitSeb/glshim,ptitSeb/glshim,ptitSeb/glshim,ptitSeb/gl4es,ptitSeb/gl4es,ptitSeb/glshim