Docs for JSON-format username + password

Author: pjdarton

docs/README.md

@@ -167,6 +167,24 @@ node {
 }
 ```
 
+### Username with Password (JSON format)
+
+A *username* and *password* pair.
+
+- Value: Valid JSON describing an object with a `username` field and and `password` field.
+- Tags:
+  - `jenkins:credentials:type` = `jsonUsernamePassword`
+
+#### Example
+
+AWS CLI:
+
+```bash
+aws secretsmanager create-secret --name 'artifactory' --secret-string '{ username: "joe", password: "supersecret" }' --tags 'Key=jenkins:credentials:type,Value=jsonUsernamePassword' --description 'Acme Corp Artifactory login'
+```
+
+Declarative and Scripted Pipeline behavior is (exactly) the same as non-JSON-format Username and Password.
+
 ### SSH User Private Key
 
 An SSH *private key*, with a *username*.