Update MakeDb documentation

[Jason Vander Heiden]

It's not documented why some records fail MakeDb. Currently, our criteria seem to be lack of V, J or junction sequence; ie, those records that IMGT failed to align. I think the criteria are fine, we just need to document it somewhere.

While we're at it, I think the list of required fields isn't accurate. JUNCTION_LENGTH and D_CALL are no longer needed, yes?

[Namita Gupta]

Added to MakeDb.py -h in 1a48aceb0417