Fix: Added missing yml lf gitattribute

[JamesHenry]

Since the gitattribute file was added in #97, my mac has automatically had unstaged changes relating to yml files.

I have never used gitattributes before, but adding a definition for yml seems to do the trick...

[eslintbot]

Thanks for the pull request, @JamesHenry! I took a look to make sure it's ready for merging and found some changes are needed:

• Pull requests with code require an issue to be mentioned at the end of the commit summary, such as (fixes #1234). Please update the commit summary with an issue (file a new issue if one doesn't already exist).

Can you please update the pull request to address these?

(More information can be found in our pull request guide.)

[dannyfritz]

Hrmm. I wonder if text=auto was causing your mac to use \r\n or something really weird like that. My impression is it will choose the system default which should b fine.

[nzakas]

LGTM