add installation instructions to docs

[whoburg]

Just received a request from a user who was confused about how to install GPfit. I've responded, but it made me realize we don't have install docs.

E-mail copied below:

Hi,

I've recently installed your GPkit python tool and I would like to test it by fitting my data as (I)SMA functions. However, when I try to run the example given for GPfit in:

http://gpfit.readthedocs.io/en/latest/examples.html

I get:

ImportError: No module named gpfit.fit

In the GPkit installation there's no reference to GPfit so I imagine this is an standalone package. How can it be installed?

Thanks in advance.