Added SequenceDictionary section.

Frank, I've filled in the Sequence Dictionaries section, let me know what you think.

Also, added a new reference to a GATK forum web-page, which gives some documentation on the sequence dictionary and explains the GATK's reliance on a common dictionary across files for comparison (as well as showing bug reports and errors reported by users who didn't maintain this distinction in their files).

More edits will be coming by, today.