Summary: This is sort of documentation.
Test Plan:
Used the plugin for Notepad++ with default indent of 4 spaces.
Opened file from `src/`, verified that it indents 2 spaces.
Opened file from `externals/`, verified that it indents 4 spaces.
Reviewers: epriestley, btrahan
Reviewed By: epriestley
CC: aran, Korvin
Differential Revision: https://secure.phabricator.com/D3865
