Update GitterIntroAkkaAkka.md

[TimMoore]

• Link to Discourse instead of the deprecated Google Group
• Link to the Lightbend Community Code of Conduct
• Update Typesafe → Lightbend

Ref: #117 for corresponding akka/dev change

[TimMoore]

I kind of like having them. The topic line of the Gitter channels link to these, and it's a good way to describe the purpose and expected usage of the channels.

On the other hand, the equivalent Play and Lagom channels don't have such files. In those channels, I've changed the welcome message instead to similar content. However this is only displayed to users when first joining the channel.

[patriknw]

in other communication we agreed to continue that discussion elsewhere, if needed