Update the README to specify which Qurro version should be installed based on your QIIME 2 version

[fedarko]

See the discussion here.

Option 1: adjust the "system requirements" section (maybe put this beforehand, and just have the system requirements section talk about python version & browsers?)

What version of Qurro should I install?

If you are using Qurro outside of QIIME 2, or if you are using Qurro within a relatively recent version of QIIME 2 (2020.11 or newer), you should install Qurro version 0.8.0 or newer. The installation commands below should install a version like this by default.

If you're using Qurro with older versions of QIIME 2 (before 2020.11), you will need to install a slightly older version of Qurro that can work with the older versions of pandas included with these QIIME 2 versions. You should install Qurro version 0.7.1.

Option 2: just add an extra parenthetical to the "system requirements" section --

System requirements

If you're using Qurro within QIIME 2, you will need a QIIME 2 version of at least 2020.11. (If you need to install Qurro into an older QIIME 2 version, you can install Qurro version 0.7.1.) [...]

The second option is more elegant, I think, but the first option is more obvious.