docs: Add an example usage of withCredentials and mocking Jenkins credentials

Author: martinreck

README.md

@@ -217,6 +217,40 @@ Please refer to the `BasePipelineTest` class for the list of currently supported
 Some tricky methods such as `load` and `parallel` are implemented directly in the helper.
 If you want to override those, make sure that you extend the `PipelineTestHelper` class.
 
+### Mocking Jenkins Credentials
+
+You can create mock credentials for use in the pipeline.
+The provided mock methods can be used to simulate sensitive data, such as access keys or user login details.
+
+```groovy
+// Jenkinsfile
+node {
+    stage('Process with credentials') {
+        withCredentials([usernamePassword(credentialsId: 'credentials-1', usernameVariable: 'USERNAME', passwordVariable: 'PASSWORD')]) {
+            echo 'User/Password: ${USERNAME}/${PASSWORD}'
+        }
+        withCredentials([string(credentialsId: 'credentials-2', variable: 'TOKEN')]) {
+            echo 'Token: ${TOKEN}'
+        }
+    }
+}
+```
+
+```groovy
+import com.lesfurets.jenkins.unit.BasePipelineTest
+
+class TestExampleJob extends BasePipelineTest {
+    @Test
+    void exampleWithCredentialsTest() {
+        // Simulates a username and password
+        addUsernamePasswordCredential('credentials-1', 'admin', 'very-strong-password')
+
+        // Simulates a secret text
+        addStringCredential('credentials-2', 'secret-token')
+    }
+}
+```
+
 ### Mocking `readFile` and `fileExists`
 
 The `readFile` and `fileExists` steps can be mocked to return a specific result for a