Document how to add custom resource types, and submit existing ones to Windmill Hub

[rudokemper]

See #1 for more context

[rudokemper]

In addition to the documentation PR, I have created these resource types and alerted the Windmill team to them:

https://hub.windmill.dev/resource_types/193/kobotoolbox_account (already existed, but just pinged them about it)
https://hub.windmill.dev/resource_types/271/arcgis_account
https://hub.windmill.dev/resource_types/272/odk_config
https://hub.windmill.dev/resource_types/273/gfw_api
https://hub.windmill.dev/resource_types/274/twilio_message_template