Add a section about dev mode and TemplateConfig to the dev docs.

docs/dev.md

@@ -153,3 +153,44 @@ Targets in the Makefile:
   necessary, and run ``golint`` and ``go test``.
 
 * ``clean``: run ``go clean``, and clean up other generated artifacts
+
+## dev mode and TemplateConfig
+
+When the controller is started with the ``-devmode``
+[command-line flag](ref-cli-options.md), it reads the ``TemplateConfig``
+Custom Resource from the kubeserver API; otherwise, ``TemplateConfig``
+is ignored, as it is not intended for production use. See the
+[docs](/docs/ref-template-cfg.md) for the authoritative reference
+on ``TemplateConfig``.
+
+Testing new templates presupposes an understanding of the source code
+-- the templates defined in [``pkg/varnish/vcl``](/pkg/varnish/vcl),
+the function maps used for ``text/template`` parsing, and the general
+design for VCL generation.
+
+The controller log and Event generation can help monitor the
+controller's implementation of ``TemplateConfig`` configuration. The
+log emits a message at level ``info`` for each template object that it
+successfully updates (meaning that the template was successfully
+parsed):
+
+```
+time="2021-02-21T17:25:39Z" level=info msg="TemplateConfig dev/viking-ingress-dev-tmplcfg: updated shard template"
+```
+
+If any template string cannot be parsed, a message at the ``error``
+level is emitted with the error message from ``text/template``, and
+the synchronization attempt for the ``TemplateConfig`` is aborted. The
+message also becomes the payload of an Event of type Warning with
+reason ``SyncFatalError``.
+
+If all of the fields specified in the ``TemplateConfig`` can be
+processed successfully, the log emits a message at level ``info``
+summarizing the fields that were updated:
+
+```
+time="2021-02-21T17:25:39Z" level=info msg="add dev/viking-ingress-dev-tmplcfg: successfully synced: TemplateConfig dev/viking-ingress-dev-tmplcfg: updated templates: [ingress acl auth reqDisp rewrite shard]"
+```
+
+The message also becomes the payload of an Event of type Info with the
+reason ``SyncSuccessful``.
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