improve install instructions (#845)
improve install instructions (#845)

Summary:
Closes: https://github.com/facebook/watchman/issues/843

Pull Request resolved: https://github.com/facebook/watchman/pull/845

Reviewed By: fanzeyi

Differential Revision: D23105084

Pulled By: wez

fbshipit-source-id: 9fb9a019c90aebc993d187c0afc9512a75eabb02

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