more plugin doc updates (#7767)

plugin/example_hello_world_test.go

@@ -0,0 +1,20 @@
+package plugin_test
+
+import (
+	"fmt"
+	"net/http"
+
+	"github.com/mattermost/mattermost-server/plugin/rpcplugin"
+)
+
+type HelloWorldPlugin struct{}
+
+func (p *HelloWorldPlugin) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	fmt.Fprintf(w, "Hello, world!")
+}
+
+// This example demonstrates a plugin that handles HTTP requests which respond by greeting the
+// world.
+func Example_helloWorld() {
+	rpcplugin.Main(&HelloWorldPlugin{})
+}