Added SequenceDictionary section.

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).
Merge pull request #3 from tdanford/tdanford/sequence-dictionaries

Added SequenceDictionary section.