Provide instructions for installing tablesorter library

Problem/Motivation

The module page and readme file don't mention that tablesorter is used.

This is causing an error on the Pool Party Taxonomy Manager module's taxonomy syncing screen.

Warning: file_get_contents(libraries/tablesorter/jquery.tablesorter.min.js): Failed to open stream: No such file or directory in Drupal\Core\Asset\JsOptimizer->optimize() (line 39 of core/lib/Drupal/Core/Asset/JsOptimizer.php).

Steps to reproduce

Proposed resolution

Add tablesorter dependency and instructions to project page and readme.

Remaining tasks

User interface changes

API changes

Data model changes