Clarify file I/O documentation

[Jason Vander Heiden]

We should clarify in the documentation how to load in AIRR Rearrangement and Change-O files. Maybe a very brief vignette that shows that you should use airr::read_rearrangement / airr::write_rearrangement for AIRR data and alakazam::readChangeoDb / alakazam::writeChangeoDb? These functions both deal with TSVs, but they have different behaviors for NA values and case conversion of sequences that are specific to each format.

See also: #87

CC: @Scott Christley

[Edel Aron]

There's now a new "File input and output" vignette