Merge pull request #466 from Relequestual/contact-us

Add contact details as required by OpenJSF

Author: Relequestual

CONTACT_US.md

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+This document exists to:
+- Document project contacts
+- Satisfy the requirements for onboarding the JSON Schema project to the OpenJS Foundation which requires documentation of project contacts for specific purposes
+
+All members of the team can be reached via the [JSON Schema slack server](https://json-schema.org/slack).
+
+## Contacts
+
+Marketing and Social Media: Benjamin Granados (@benjagm)\
+Infrastructure: Ben Hutton (@relequestual)\
+Legal/governance help: Ben Hutton (@relequestual)