Improve arcanist install documentation
Summary:
See https://github.com/facebook/arcanist/issues/21, https://github.com/facebook/arcanist/issues/22.
- Provide more detailed install instructions.
- Provide advice on managing team installs.
- Provide information on libphutil path resolution.
- Provide OSX-specific documentation.
- Update some of the documentation to reflect evolution of the tools.
Test Plan: Generated and read documentation.
Reviewers: btrahan
Reviewed By: btrahan
CC: jmhsieh, aran, epriestley
Differential Revision: https://secure.phabricator.com/D1990