add documentation page

Author: Thomas Poignant

docs/notifier/custom.md

@@ -0,0 +1,27 @@
+# Custom Notifier
+To create a custom notifier you must have a `struct` that implements the [`ffnotifier.notifier`](https://pkg.go.dev/github.com/thomaspoignant/go-feature-flag/ffnotifier/notifier) interface.
+
+```go linenums="1"
+import (
+	"fmt"
+	ffclient "github.com/thomaspoignant/go-feature-flag"
+	"github.com/thomaspoignant/go-feature-flag/ffnotifier"
+	"sync"
+)
+
+// Your config object to create a new Notifier
+type CustomNotifierConfig struct{}
+
+// GetNotifier returns a notfier that implement ffnotifier.Notifier
+func (c *CustomNotifier) GetNotifier(config ffclient.Config) (ffnotifier.Notifier, error) {
+	return &CustomNotifier{}, nil
+}
+
+type CustomNotifier struct{}
+func (w *CustomNotifier) Notify(cache ffnotifier.DiffCache, waitGroup *sync.WaitGroup) {
+	defer waitGroup.Done() // don't forget this line, if you don't have it you can break your notifications
+	
+	// ...
+	// do whatever you want here
+}
+```

mkdocs.yml

@@ -82,4 +82,5 @@ nav:
       - 'notifier/index.md'
       - 'notifier/slack.md'
       - 'notifier/webhook.md'
+      - 'notifier/custom.md'
   - 'faq.md'