[README] Add Gitter chat badge
[README] Add Gitter chat badge

I'm not sure how long we'll support this, but I've been keeping this open
for now, and it's better for some conversations than github issues.
sachgits/perseus,learningequality/perseus,ariabuckles/perseus,ariabuckles/perseus,learningequality/perseus,iamchenxin/perseus,iamchenxin/perseus,alexristich/perseus,chirilo/perseus,ariabuckles/perseus,daukantas/perseus,alexristich/perseus,daukantas/perseus,ariabuckles/perseus,chirilo/perseus,abe4mvp/perseus,abe4mvp/perseus,sachgits/perseus