Problem

There are no docs for how to use custom percentiles, and it's also tough for
Java users to construct Seqs.

Solution

Add some docs for custom percentiles, and some nice facilities for Java users to
use them.

JIRA Issues: CSL-10686

Differential Revision: https://phabricator.twitter.biz/D620425

• Commit cddcbf8
• Compare 35abeca..cddcbf8
• Branch develop

finaglehelper