Added updates per JIRA issue PREOPS-3791

[DouglasLeeTucker]

Variations of both proposed updates from PREOPS-3791 have been implemented:

Proposed Update 1

Update: "Packages can be installed in a ~/local directory. (If you do not already have a ~/local directory, you can create one via the command mkdir ~/local)."   To say something like: "Packages can be installed in a ~/local directory (or your preferred equivalent, if you have already installed other user packages into another directory in your user area on the RSP). If you do not already have a ~/local directory (or your preferred equivalent), you can create one via the command mkdir ~/local)."

Proposed Update 2

Add a section describing how to add a path to one's PATH and PYTHONPATH environmental variables if, for some reason, a package's commands are not being "picked up" either on the command line or in a notebook. This should be taken care of automatically by "pip install --user " or "python setup.py install --home=" in most cases.

For example, sometimes when downloading packages from GitHub, a full official install is not necessary and commands like this in the equivalent of the .bashrc, and/or ~/notebooks/.user_setups file, work fine:

# Setup DECam_PGCM environment...
export DECAM_PGCM_DIR=/usrdevel/dp0/dtucker/GitHub/DECam_PGCM
export PATH=${DECAM_PGCM_DIR}/bin:${PATH}
export PYTHONPATH=${DECAM_PGCM_DIR}/python:${PYTHONPATH}

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I followed pretty closely to the text of the original Proposed 1.

For Proposed Update 2, I used for a concrete example the case where the user downloads a bleeding edge of Jake Vanderplas's astroML package from github.com as a working copy -- possibly with the idea of modifying some of the downloaded astroML code -- and so does not wish to immediately install it in a regular installation directory.

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[MelissaGraham]

OK to merge after the two small fixes, no need to re-review.