Document constraints.txt usage.

[loechel]

I have documented the corresponding constrains.txt usage with tox, as it work similar to requirements.txt but was not documented.

Here's a quick checklist of what we'd like you to think off:

• Add yourself to CONTRIBUTORS;
• make a descriptive Pull Request text (it will be used for changelog)

[obestwalter]

typo: varaiable

[obestwalter]

This would have to be deps = -cconstrains.txt

[obestwalter]

Hi @loechel thanks for the PR. I guess you mean this: https://pip.pypa.io/en/stable/user_guide/#constraints-files?

If yes: this is nothing that is implemented as such - it is basically a side effect of the way how deps is implemented. deps passes the -<someflag>whatever.txt through to pip, so you can theoretically use any flag that would make sense to pip there.

Maybe it would be better to explain that in above section documenting -r that mentions that it is also possible to pass e.g. the -c flag with a constraints file?

[obestwalter]

s/constrains/constraints/g

[loechel]

@obestwalter thanks for the feedback, I have adjusted the documentation. I hope it is better now and ready to merge.