Add troubleshooting to documentation

[vzhz]

To make it easier for new users to troubleshoot their invalid RSS feeds, I suggest listing

1. resources for validating your RSS feed and

2. any errors that can be anticipated, given the parser, that may not be found by validating tools.

I'd be happy to dig in and work on this.
If there's interest in this, would love some input about a location for the troubleshooting section. My thought is that the it would make the most sense where a frustrated user might go, the primary project README, but I see that the primary mostly motivation to begin writing and that the dev docs are in a separate folder, so I'm wondering if it would be most consistent to keep trouble shooting in another file that is linked from the primary README?