Update installation instructions to mention conda requirement

[twiecki]

If you try to pip install pymc3, like our readme instructs, you will have a bad time on most platforms because:

• You will have a hard time linking your system compilers to theano
• It will be super slow because mkl will not be installed

So really, it is required to at least install the dependencies of theano and pymc3 via conda/conda-forge and we should update our instructions accordingly.

[CloudChaoszero]

I can take this on @twiecki.

[twiecki]

Thanks @CloudChaoszero!