docs: colorize Discord badge & add icon in README

[wescpy]

Update "chat" badge on README to make it more visible for visitors; changing the look from

... to ...

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[terrytangyuan]

It might be better to keep these in one line? Otherwise on mobile it looks like this

[terrytangyuan]

Please sign the individual CLA for now

[wescpy]

FIXED & SIGNED

[terrytangyuan]

Thanks!

[terrytangyuan]

We were told to hold off on merging anything for the rest of the week (requested from Meta). Will merge next week.

[wescpy]

SGTM (sounds good to me)