Updated troubleshooting to add permissions example (#262)

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Added another case that can cause `auth` to fail, helpful to users on
day one of using the tool and trying to understand what's happening.

Fixes
#260 (comment)

docs/TROUBLESHOOTING.md

@@ -35,7 +35,7 @@ further debug:
     libraries. Please note that we do not have control over actions outside of
     `google-github-actions`.
 
-If your workflow _fails_ after adding the the step to generate an access token,
+If your workflow _fails_ after adding the step to generate an access token,
 it likely means there is a misconfiguration with Workload Identity. Here are
 some common sources of errors:
 
@@ -55,6 +55,15 @@ some common sources of errors:
     **number**. Workload Identity Federation does not accept Google Cloud
     Project IDs.
 
+1.  Ensure that you have the correct `permissions:` for the job in your workflow, per
+    the [usage](../README.md#usage) docs, i.e.
+
+    ```yaml
+    permissions:
+      contents: 'read'
+      id-token: 'write'
+    ```
+
 1.  Ensure you have created an **Attribute Mapping** for any **Attribute
     Conditions** or **Service Account Impersonation** principals. You cannot
     create an Attribute Condition unless you map that value from the incoming