docs: base64 decode and encode (#182)

Signed-off-by: Attila Mészáros <[email protected]>

Author: csviri

docs/reference.md

@@ -250,6 +250,28 @@ or for Deployment:
 `glue.operator.resource-label-selector.apps/v1#Deployment=mylabel=samplevalue`
 
 
+## Extending qute templating engine
+
+[Qute templating engine](https://quarkus.io/guides/qute) is very flexible.
+We extend it only we two additional functions, to decode and encode base64 values. More might come in the future.
+You can call these on every string of byte array, using `decodeBase64` and `encodeBase64` keywords.
+
+```yaml
+ resourceTemplate: |
+        apiVersion: v1
+        kind: ConfigMap
+        metadata:
+          name: my-secret-copy
+          namespace: namespace-a
+        data:
+          {#for entry in secret-to-copy.data}
+            {entry.key}: {entry.value.decodeBase64}
+          {/for}
+
+```
+
+See the full example [here](https://github.com/java-operator-sdk/kubernetes-glue-operator/blob/main/src/test/resources/glue/CopySecretToConfigMap.yaml).
+
 ## Implementation details and performance
 
 Informers are used optimally, in terms of that, for every resource type only one informer is registered in the background. Event there are more `Glue` or `GlueOperator`

src/test/resources/glue/CopySecretToConfigMap.yaml

@@ -1,3 +1,4 @@
+# Sample copies content of a secret from namespace-b to a config map in namespace-b.
 apiVersion: io.javaoperatorsdk.operator.glue/v1beta1
 kind: Glue
 metadata: