DOC: Add more documentation on choosing a python on install (#196)

* DOC: Add notes on install instructions

Author: gerrymanoim

README.rst

@@ -47,6 +47,12 @@ To install for development:
 
 Otherwise, ``pip install libpy``, making sure ``CC`` and ``CXX`` environment variables are set to the the right compiler.
 
+**Note**: The installation of ``libpy`` will use the ``python`` executable to
+figure out information about your environment. If you are not using a virtual
+environment or ``python`` does not point to the Python installation you want
+to use (checked with ``which python`` and ``python --version``) you must
+point to your Python executable using the ``PYTHON`` environment variable,
+i.e. ``PYTHON=python3 make`` or ``PYTHON=python3 pip3 install libpy``.
 
 Tests
 -----

docs/source/install.rst

@@ -32,6 +32,13 @@ To install for development:
 
 Otherwise, ``pip install libpy``, making sure ``CC`` and ``CXX`` environment variables are set to the the right compiler.
 
+.. note::
+   The installation of ``libpy`` will use the ``python`` executable to
+   figure out information about your environment. If you are not using a virtual
+   environment or ``python`` does not point to the Python installation you want
+   to use (checked with ``which python`` and ``python --version``) you must
+   point to your Python executable using the ``PYTHON`` environment variable,
+   i.e. ``PYTHON=python3 make`` or ``PYTHON=python3 pip3 install libpy``.
 
 Tests
 -----