Rule: img tags must have alt attributes

Hello! Please let me know if this is the way I should do it. If this issue is cool, I’ll add more rule proposals.

Bad:

<img src="hello.jpg">
<img src="//cdn.verylongurl.com/1234/bucket/hello.jpg">

Good:

<img src="hello.jpg" alt="">
<img src="logo.jpg" alt="Your Website Name">
<img src="save-icon.jpg" alt="Save draft">
<img src={{userGeneratedUrl}} alt={{userGeneratedCaption}}>

Screen readers read images as text. Without alt attributes, screen readers fallback to reading the full src for an image out loud. With CDNs, that will get very annoying 😃 To mitigate, always provide alt attributes for images. The value of the attribute should present the content and function of the image. Blank alt attributes are preferred if the image is purely presentational, but if it displays information (e.g. an image of text), transcribe the text; or if it’s tied to functionality, write what it does as if it were a button. Ref: http://webaim.org/techniques/alttext/

[ghost]

For the record: opted to ban all empty alt tags, see PR for more details.

[ghost]

Would love to. Feel free to assign my way.