This doc is no longer fetched by docs.nextstrain.org¹, so the warning is
no longer needed.

¹ nextstrain/docs.nextstrain.org@62e4a3a

Commands used:

    pandoc -f markdown -t rst --wrap=none docs/introduction/install.md -o docs/introduction/install.rst
    rm docs/introduction/install.md

- Add a link to the Nextstrain installation guide
- Add table of contents
- Split into "user" and "developer" sections
- Fix Conda environment setup command
- Remove note on bash/env requirement. Don't think it's necessary.

Instead of having duplicate instructions getting out of sync.