Add the chapter "Varnish as Ingress" to the front-page README.

README.md

@@ -90,3 +90,47 @@ $ go get -d github.com/slimhazard/gogitversion
 $ cd $GOPATH/src/github.com/slimhazard/gogitversion
 $ make install
 ```
+# Varnish as a Kubernetes Ingress
+
+Since this project is currently in its early stages, the implementation of
+Ingress definitions for a Varnish instance is subject to change as development
+continues. Presently, an Ingress is realized by loading a
+[VCL](https://varnish-cache.org/docs/trunk/reference/vcl.html) configuration
+that:
+
+* defines
+  [directors](https://varnish-cache.org/docs/trunk/users-guide/vcl-backends.html#directors)
+  that correspond to Services mentioned in the Ingress definition
+  * This is currently hard-wired as the round-robin director.
+* defines
+  [backends](https://varnish-cache.org/docs/trunk/users-guide/vcl-backends.html)
+  corresponding to the Endpoints of the Services. These are assigned to the
+  director defined for the Service.
+  * Endpoint definitions for a Service are obtained from an API query
+    when the VCL configuration is generated, so that the assignments
+    of Endpoints is current at VCL generation time.
+  * The Varnish backend definitions currently only include the Endpoints'
+    IP addresses and ports. There is presently no means to define other
+    features of a backend, such as health checks and timeouts.
+* generates VCL code to implement the routing of requests to backends
+  based on the Host header and/or URL path according to the IngressRules
+  given in the Ingress definition.
+  * Varnish may cache responses according to its default rules for
+    caching, and of course cache hits are delivered without routing the
+    requests further.
+  * In case of a non-cache-hit (miss or pass), the Host header and/or
+    URL path is evaluated according to the IngressRules, and matching
+    requests are assigned to the director corresponding to the matched
+    Service.
+  * The director in turn chooses a backend corresponding to an Endpoint
+    according to its load balancing algorithm (currently only round-robin).
+  * If the request does not match any Service according to the
+    IngressRules, then:
+    * If a default Backend (a Service) was defined in the IngressSpec,
+      then the request is assigned to the corresponding director.
+    * Otherwise, a synthetic 404 Not Found response is generated by
+      Varnish.
+  * If there is no valid Ingress definition (none has been defined
+    since the Varnish instance started, or the only valid definition
+    was deleted), then Varnish generates a synthetic 404 Not Found
+    response for every request.