Adding how to install fly cli

[ChrisJBurns]

I've just followed the guide and used the Concourse CI to download the mac binary for the fly CLI. However it doesn't say to insert into /usr/local/bin directory. Perhaps it should, I know from experience to put it there, but someone new may not know this.

Just a suggestion.

[taylorsilva]

Recent docs changes cover this I think. The install should be much easier to follow for users not familiar with the terminal