Document the need to do "git submodule update --init" after cloning

Summary:
After rP2a39fd09ebe7f4fc8cd2ab0b39bbb0e466f357c3, you need to run this command
after cloning Phabricator.

Git, world's hardest revision control system

Test Plan:
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Reviewed By: rm
Reviewers: tomo, tuomaspelkonen, jungejason, aran, rm
CC: aran, rm, epriestley
Differential Revision: 252