Replaced readme slack link with discord link

[mhuggins]

What current issue(s) from Trello/Github does this address?
N/A

What problem does this PR solve?
The README still linked to Slack instead of Discord.

How did you solve this problem?
I replaced the Slack link and text with Discord link and text.

How did you make sure your solution works?
I clicked the link.

Are there any special changes in the code that we should be aware of?
No.

Is there anything else we should know?
No.

• Unit tests written?
  N/A

[coveralls]

Coverage remained the same at 39.239% when pulling 851077d on neoverse:slack-discord-readme-link into 9b6a6f2 on CityOfZion:dev.

[slipo]

Thanks!