Instructions for installing on FreeBSD

[irphilli]

I have completed a port for installing fetch on FreeBSD and would like to update the docs to include FreeBSD installation instructions. There's one caveat that I could use some feedback on... FreeBSD already has a base utility called fetch, so the port is installing this project as go-fetch. As far as I can tell, there are no side effects to this. Let me know if you know of any "gotchas" here! If not, I will move forward with formally submitting my port to FreeBSD, along with a PR here to update the docs. Thank you!