user doc: Add xref to Syndesis community doc for developing connectors
If {prodname} does not provide a connector for the application or
service that you want to connect to in an integration,
an experienced developer can code an extension that contributes a
-new connector to {prodname}.
+new connector to {prodname}. This documentation provides an introduction
+to developing a connector extension. For details about developing a
+connector, see
+link:https://syndesis.io/docs/connectors/[Developing Syndesis connectors]
+on the Syndesis community site.
[IMPORTANT]
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