document EarnBatch limitation

[rorymc]

We've been using kin-node for more than a year, and just discovered an undocumented limit in the "EarnBatch" code.

Had to dig into the kin-node code, to discover there was a limit.

Please help add implementation notes to kin-node documentation, so we know how to properly use it without errors:

• Error: An EarnBatch must not contain more than 15 earns.