Add Azure to readme

[ytbryan]

previously rails/activestorage#86

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @pixeltrix (or someone else) soon.

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[rails-bot]

Warning

• Pull requests are usually filed against the master branch for this repo, but this one is against active-storage-import. Please double check that you specified the right target!