Provide setup instructions for reducing minimum index word length

Summary:
The MySQL MyISAM fulltext backend defaults to a minimum length of 4, but many
3-letter words are useful search terms. Provide instructions for configuring
MySQL to reduce the auto-stopword length.

Test Plan:
Followed instructions, searched for a 3-letter term and got a hit.

Reviewed By: jungejason
Reviewers: aran, tuomaspelkonen, jungejason
CC: aran, jungejason
Differential Revision: 471