Add key to 'Support' section in README?

[btamayo]

Hi, I find the 'Support' section in the current README lacking a key for _, X, etc. I'm presuming X means that there is support for that item but perhaps we can add a key or change it to a checkmark instead?

[levithomason]

I had tried this previously, but they were hideous. Found I could copy the rendered ✓ and paste it in, not bad:

https://github.com/TechnologyAdvice/stardust#support

Thanks