Addresses #8177, Readme badges

[epintozzi]

Added badges for: Travis, Appveyor, Codecov, pypi, gitter, and depsy

[anntzer]

I would suggest a plain text gitter link next to the mailing-list section.

You can also update the pypi link so that it points directly to pypi rather than via a redirection.

PS for whoever is reviewing this: you can check how it looks like at https://github.com/epintozzi/matplotlib/tree/readme_badges.

[tacaswell]

Can we be vain and not include codecov? 😈

Agree on including text link to gitter with the mailing lists

the 'view' button in the review page will also show you the rendering.

[tacaswell]

Minor preference for a text link to gitter, but fine with this going in as-is.

Please backport to 2.0.0-doc

[dstansby]

I'll consider that a positive review @tacaswell, and merge!

[dstansby]

This looks like your first contribution to Matplotlib @epintozzi, thanks a lot!

[dstansby]

Backported to 2.0.0-doc via. 6e52c51