Add slack channel

[benaadams]

Improves community x8 and support x2 and overall .NET throughput 15 fold on the techempower tests 😉

[dnfclas]

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[muratg]

cc @glennc

(I'm totally stealing "x8" 😄 )

[Eilon]

We're having some discussions on this.

[spboyer]

@Eilon let me know if there is anything I need to do to help the discussion along, or make it easier.
@shanselman @DamianEdwards

[Daniel15]

Why Slack rather than JabbR or Gitter? .NET Core is using Gitter (eg. https://gitter.im/dotnet/corefx), and Slack doesn't scale to larger room sizes, their product is only focused on smaller teams.

[benaadams]

@Daniel15 gitter is already on page, this just adds the slack next to it, channel is already at 1700 members an it seems ok. JabbR was having having major responsiveness issues else we'd still be there 😢

[glennc]

Closing this, re-open if it still needs to happen but I think @shanselman has settled on only Gitter.