setting PATH in env vars section

[jrcain-usgs]

Licensing

Adds instructions on how to set your PATH when installing ISIS.

Fixes #128

This project is mostly composed of free and unencumbered software released into the public domain, and we are unlikely to accept contributions that are not also released into the public domain. Somewhere near the top of each file should have these words:

This work is free and unencumbered software released into the public domain. In jurisdictions that recognize copyright laws, the author or authors of this software dedicate any and all copyright interest in the software to the public domain.

• I dedicate any and all copyright interest in this software to the public domain. I make this dedication for the benefit of the public at large and to the detriment of my heirs and successors. I intend this dedication to be an overt act of relinquishment in perpetuity of all present and future rights to this software under copyright law.

[Kelvinrr]

I don't know if this is addressing the issue.

When installing ISIS, you do not have to set your PATH variable. I think what @michaelaye was getting at was when doing development installs or legacy (non conda) installs. Or when you install ISIS in one environment and want to access it when the env is not activated.

[Kelvinrr]

e.g., here, this would not accomplish much since $CONDA_PREFIX/bin is already in your PATH on activation.

Idk if we want to encourage people to set their PATH to another conda env after install, if you need that I imagine stacking would be better?