Clarifying a few steps in the install

[jasonhemann]

I ran across a few difficulties in installation and setup. Without these additions, I was seeing e.g.

Collecting rpy2
  Downloading rpy2-3.4.4.tar.gz (187 kB)
     |████████████████████████████████| 187 kB 9.3 MB/s 
    ERROR: Command errored out with exit status 1:
     command: /Users/jhemann/Documents/Trinity/tyrell/venv/bin/python3.9 -c 'import io, os, sys, setuptools, tokenize; sys.argv[0] = '"'"'/private/var/folders/lv/300nvm1d6lq_dmww4c8h9z680000gn/T/pip-install-jzjvdvx6/rpy2_8aca3d4c44d8453da3af9292c3e428ae/setup.py'"'"'; __file__='"'"'/private/var/folders/lv/300nvm1d6lq_dmww4c8h9z680000gn/T/pip-install-jzjvdvx6/rpy2_8aca3d4c44d8453da3af9292c3e428ae/setup.py'"'"';f = getattr(tokenize, '"'"'open'"'"', open)(__file__) if os.path.exists(__file__) else io.StringIO('"'"'from setuptools import setup; setup()'"'"');code = f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /private/var/folders/lv/300nvm1d6lq_dmww4c8h9z680000gn/T/pip-pip-egg-info-y20nejf7
         cwd: /private/var/folders/lv/300nvm1d6lq_dmww4c8h9z680000gn/T/pip-install-jzjvdvx6/rpy2_8aca3d4c44d8453da3af9292c3e428ae/
    Complete output (4 lines):
    WARNING: The wheel package is not available.
    WARNING: The wheel package is not available.
    Error: rpy2 in API mode cannot be built without R in the PATH or R_HOME defined. Correct this or force ABI mode-only by defining the environment variable RPY2_CFFI_MODE=ABI
    ['cffi>=1.10.0', 'jinja2', 'pytz', 'tzlocal']
    ----------------------------------------
WARNING: Discarding https://files.pythonhosted.org/packages/cb/a5/354fdfc13ac30fd496c100a66587bbf7f0adcd320fa751311566f198bdbd/rpy2-3.4.4.tar.gz#sha256=e198862701bfc6d3da4a2fcf18910d71521b385283abdb2121e07b874add8e50 (from https://pypi.org/simple/rpy2/). Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.