Improve documentation of OTF feature files

It seems that the correct keyword for character substitution is "subsitute", not "sub". This concurs with the spec at http://www.adobe.com/devnet/opentype/afdko/topic_feature_file_syntax.html#6.b.i and the working example at http://compgroups.net/comp.text.tex/fontspec-+-lualatex-using-a-featurefile/121254

Add a note about naming of feature files. (If I have uppercase letters in a feature file name, it doesn't work.)