add section on how/where to report bugs and suggestions

[crotwell]

FDSN StationXML and Web Services pages provide links to github for reporting issues, but SourceId doesn't. Would be helpful if people that find problems can easily find where to report them.

Not sure that SourceIds needs a fdsn.org landing page instead of the going directly to readthedocs, but that might be the easiest way to have a place to put a github link.

Longer term, having a "where to report issues" section in the actual docs would be nice.