Adding New Filters Documentation

Hello,

I wanted to confirm, as suggested in issue #46, that modifying the FILTER_LIST, magsun.dat, and filter_lambda_eff.dat files is important when adding new filters to python-fsps. My code gave non-physical results until I added information for my custom filters into these files.

Would it be possible to add documentation about adding new filters to the python-fsps documentation page, since the filter-adding process is different than that for FSPS? This might help others avoid this issue in the future, especially those who have less experience with github.

Thank you!