update docs to conda install?

[taylorreiter]

Should we update the sourmash tutorial documentation to use conda? Currently recommended install can be found here: https://github.com/dib-lab/sourmash/blob/master/doc/tutorials.md

[luizirber]

I think pip is fine, but we should recommend using pip install --pre sourmash instead of pointing to the master branch (pip install -U https://github.com/dib-lab/sourmash/archive/master.zip)

[taylorreiter]

I like how conda takes care of dependencies though, esp if someone isn't using a blank ubuntu image. Maybe we could have both?

[ctb]

Changed in #631. It's just easier to use conda on a blank system!

[ctb]

This should be done now, with #631 & #592.