Fix cayley install instruction
Fix cayley install instruction

Summary: go get didn't actually install cayley like the instruction assumes it does, so I provided a link for the user to manually install cayley however they want.

Test Plan: None

Reviewers: #core_team, zarko

Reviewed By: #core_team, zarko

Differential Revision: https://phabricator-dot-kythe-repo.appspot.com/D933

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