Add fix for custom domain overwrite deploy bug

[christianboyle]

If this CNAME record doesn't exist in /static when you run the deploy script the custom domain user setting in GitHub will be reset to https://<username>.github.io.

Bug and fix described here: tschaub/gh-pages#213

[travis-r6s]

@christianboyle Sorry, my bad - the MD linter wants to have a language tag. I guess you could just add txt?

[christianboyle]

I was going to try that, but didn't see txt as an option here: https://www.rubycoloredglasses.com/2013/04/languages-supported-by-github-flavored-markdown/

Are any of these more appropriate than js?

Edit: Nevermind, I see txt now, will fix: https://github.com/github/linguist/blob/master/lib/linguist/languages.yml#L5712

[travis-r6s]

Wow, that's a super long list, and useful for future reference! Thanks