Document troubleshooting/help

[rogerbinns]

Please add a section to the documentation saying how to troubleshoot issues and then get help with them.

For example:

1. run sigal build -fvd -n 1 to get diagnostic info
2. create new photos directory containing only last photo mentioned
3. rerun sigal build -fvd -n 1
4. put output into a gist/paste
5. write to mailing list

[saimn]

Good idea, thanks !