Point to discord instead of gitter

[jochem-brouwer]

Our repo's currently point to gitter instead of discord. Let's change that.

[ryanio]

sounds good! I already created a badge if everyone likes it:

[![Discord][discord-badge]][discord-link]

[discord-badge]: https://img.shields.io/static/v1?logo=discord&label=discord&message=Join&color=blue
[discord-link]: https://discord.gg/TNwARpR

[holgerd77]

Yes, makes sense, Discord badge looks also nice. 👍

[evertonfraga]

My criteria was: every non-archived project that had commits >=2019 and had a gitter badge.

There are about 3 projects that do not have any sort of chat badge, I can add to them later.

#65
ethereumjs/ethashjs#28
ethereumjs/ethereumjs-abi#85
ethereumjs/ethereumjs-util#273
ethereumjs/ethereumjs-monorepo#854
ethereumjs/ethereumjs-client#141
ethereumjs/ethereumjs-testing#50
ethereumjs/ethereumjs-wallet#134
ethereumjs/ethereumjs.github.io#7
ethereumjs/ethereumjs.github.io#8

[holgerd77]

This has been addressed (at least for the very most part, not sure if there are 1-2 occurrences left, but these can be done on occasion).

Will close.