Update README with installation instructions

[alephcero]

README should contain installation instructions for environments created with virtualenv and conda. For virtualenv could be as follow:

virtualenv venv --python=python3.7
source venv/bin/activate
pip install -r requierements.txt
cd synthpop/
python setup.py develop
ipython kernel install --user --name=synthpop
Add to /venv/bin/activate the following line: export CENSUS='1234ebcf' 

[PyMap]

Here my notes, https://github.com/urbansim/synthpop-planning/issues/3