Added slack greetbot

Author: Relequestual

slack-greetbot

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+Hey {{Joiner}}, Welcome to the JSON Schema Slack Server!
+
+Whatever reason you chose to join us, we want you to know, you're welcome!
+We are in the process of formalising our Code of Conduct, but we already subscribe to the IETF CoC, BCP54/RFC7154: <https://www.rfc-editor.org/rfc/rfc7154.html>.
+Be nice, be kind, be respectful.
+
+If you need help, or you're looking to get stuck in with the JSON Schema community, you're in the right place.
+
+This server includes the specification authors, many implementers, and a large number of helpful individuals from the community.
+
+If you're looking for an implementation of JSON Schema, you should check out the <https://json-schema.org/implementations.html|implementations page>.
+
+If you're looking to learn the basics of JSON Schema, do check out our <https://json-schema.org/learn|learning resources>.
+
+If you want to ask questions or discuss JSON Schema, the #general channel is the right place.
+We also recommend you join the #announcements channel, which is low traffic.
+
+Here are some of our other channels and why you might want to join them.
+
+If you have any questions about our community, please feel free to ask in #general, or DM {{@relequestual}}.
+
+#implementers - Those implementing the specification and questions about implementation details
+#specification - Potential changes to the JSON Schema specification
+#tests - Development of the official test suite
+#community - Development of the JSON Schema community and organisation
+#in-the-wild - Posting of any references to uses of JSON Schema by others for tracking
+
+#watercooler - Space to chat about anything off topic or just catch up
+#z-random - Anything that's really far off topic
+
+#stack-overflow - Automated feed of StackOverflow questions tagged with JSON Schema
+#github - Automated feed of Github Issues and Pull Requests