Adds installation instructions for FreeBSD to README

[nrekretep]

Background

I use vegeta on FreeBSD and just realized that there is the latest version available as package to be installed with the freebsd package manager.

Maybe this is worth sharing in the README.

Checklist

• [x] Git commit messages conform to community standards.
• [x] Each Git commit represents meaningful milestones or atomic units of work.
• [x] Changed or added code is covered by appropriate tests.

[tsenart]

Merged in 96e735dba533487fa389a85a3fceb76bd2081717