Provide user with information about how to install the command

[mrmanc]

For people (like me) who are not familiar with Go.

[palmerabollo]

I ran go get github.com/mrmanc/spectro/normal github.com/mrmanc/spectro/pacemaker github.com/mrmanc/spectro/spectro following the instructions in the README but I still get a "command not found" error when I try to run spectro.