warn about 'rmd' file extension in epsiodes folder

[tobyhodges]

A point of confusion encountered in the Collaborative Lesson Development Training pilot: pages did not build from source files with the .rmd extension (the files had to be renamed to use .Rmd first). This is an easy mistake to make and I wonder if we can have a validation check that warns the user about this file extension and suggests the fix that they will probably want to make?

[zkamvar]

Actually, I think it would be better just to allow the "rmd" file extension since there is precedent for allowing it.