Add comment for reverse complement behaviour

//When a startOffset or endOffset is in a block that is reverse complemented, the reverse complement
//operation is performed first.  So if the thread has startOffset = 1 and endOffset = 3 in a block 
//with sequence ACCTA the thread sequence is CC, but if the block is reverse complemented to TAGGT
//the thread sequence with the same offsets is AG.
Merge pull request #5 from richarddurbin/master

Add comment for reverse complement behaviour