Update readme to include conda env config vars option (#4515)

* Update readme to include conda config var option

First step for issue #4499

Updates readme step 7 of install to include steps for using `conda env
config vars` to set environment variables as option 2.

there are a few things to work out:

1. Is ISISTESTDATA actually required?
2. What about ALEDATA?
3. Update miniconda/anaconda version mentioned to a more recent release
4. Remove mentions of isis version < 4.2.0, as older readmes contain
   install directions
5. Verify I got the update command correct.
6. See why the markdown doesn't indent for options 1 and 2.

* Update README.md

removing direct mentions of ISIS 4.2.0 from env steps, added isis 5.0.0 readme link.

* remove direct mention of ISISTESTDATA from readme