Merge pull request 99designs#573 from 99designs/plugin-docs

Very rough first pass at plugin docs

docs/content/reference/plugins.md

@@ -0,0 +1,62 @@
+---
+linkTitle: Plugins
+title: How to write plugins for gqlgen
+description: Use plugins to customize code generation and integrate with other libraries
+menu: { main: { parent: 'reference' } }
+---
+
+Plugins provide a way to hook into the gqlgen code generation lifecycle. In order to use anything other than the
+default plugins you will need to create your own entrypoint:
+
+
+## Using a plugin
+```go
+// +build ignore
+
+package main
+
+import (
+	"flag"
+	"fmt"
+	"io/ioutil"
+	"log"
+	"os"
+	"time"
+
+	"github.com/99designs/gqlgen/api"
+	"github.com/99designs/gqlgen/codegen/config"
+	"github.com/99designs/gqlgen/plugin/stubgen"
+)
+
+func main() {
+	cfg, err := config.LoadConfigFromDefaultLocations()
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "failed to load config", err.Error())
+		os.Exit(2)
+	}
+
+
+	err = api.Generate(cfg, 
+		api.AddPlugin(yourplugin.New()), // This is the magic line
+	)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err.Error())
+		os.Exit(3)
+	}
+}
+
+``` 
+
+## Writing a plugin
+
+There are currently only two hooks:
+ - MutateConfig: Allows a plugin to mutate the config before codegen starts. This allows plugins to add
+    custom directives, define types, and implement resolvers. see
+		 [modelgen](https://github.com/99designs/gqlgen/tree/master/plugin/modelgen) for an example
+ - GenerateCode: Allows a plugin to generate a new output file, see
+    [stubgen](https://github.com/99designs/gqlgen/tree/master/plugin/stubgen) for an example
+
+Take a look at [plugin.go](https://github.com/99designs/gqlgen/blob/master/plugin/plugin.go) for the full list of 
+available hooks. These are likely to change with each release.
+
+