Add auto fixer for jsx-space-before-trailing-slash

[heatherbooker]

Hi,

This PR adds an auto fixer for the jsxSpaceBeforeTrailingSlash rule, along with a test and readme update.

The auto-fix uses appendText to add a single space before offending closing tags.

Let me know if it's missing anything! Shoutout to @zapo for co-driving to do this! 🙌

[palantirtech]

Thanks for your interest in palantir/tslint-react, @heatherbooker! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[heatherbooker]

I signed the contributor's agreement but it seems it might not recognize it because the main email for my account is what the system used on the contract, but this commit is associated with a different email address (also on this account). 🤔

Fixed it!

[adidahiya]

nice, thanks @heatherbooker