Explain how to extract credentials/settings

docs/credentials/extract_credentials.md

@@ -0,0 +1,11 @@
+Extract credentials
+===================
+
+Credentials and encrypted settings can be extracted using the following snippet:
+
+```python
+# awx-manage shell_plus
+>>> from awx.main.utils import decrypt_field
+>>> cred = Credential.objects.get(name="my private key")
+>>> decrypt_field(cred, "ssh_key_data")
+```