Add necessary steps to setup instructions

[lucassz]

I noticed while setting up a local installation of PolicyBrain that these steps are definitely needed (to install prerequisite packages TaxBrain and B-Tax).

[hdoupe]

@lucassz Thanks for opening up this PR. The instructions can always be improved.

I have a question about this line:

conda install -c ospc taxcalc btax

These packages should be installed in the python_env_build.sh script. Were they not in the pb_env conda environment?

[lucassz]

These packages should be installed in the python_env_build.sh script. Were they not in the pb_env conda environment?

@hdoupe I remember having to perform that step manually even in the environment, but you are right that they should have been installed according to the condo-requirements.txt file.

[hdoupe]

What's the output when you run source activate pb_env && conda list taxcalc?

[lucassz]

It seems to show up fine in that case, so it's possible I was doing something wrong before.

[hdoupe]

Ok, cool. In that case, you can remove the extra conda install step and we can merge this PR with the link addition.

[lucassz]

@hdoupe Done.

[hdoupe]

LGTM. Thanks for the contribution @lucassz