add markdown file to help with debugging

Signed-off-by: Lachlan Roberts <lachlan.p.roberts@gmail.com>

debugging.md

@@ -0,0 +1,34 @@
+# Google App Engine Java Debugging Guide
+
+## Jetty Debug Logging
+
+To enable debug logging in your web application, add a JUL `logging.properties` file under your application's `WEB-INF` directory.
+
+In your `appengine-web.xml` file, specify the location of the logging file as a system property:
+
+```xml
+<system-properties>
+  <property name="java.util.logging.config.file" value="WEB-INF/logging.properties"/>
+</system-properties>
+```
+
+For runtimes using the Jetty 9.4 code path (Java 8 - Java 17), there is a known issue where debug logging will not be output unless full debug logging is enabled with `.level=ALL`.
+
+In the Java 21 runtime with Jetty 12, more granular debug logging is available. For example:
+```
+.level=INFO
+org.eclipse.jetty.session.level=ALL
+org.eclipse.jetty.server.level=ALL
+```
+
+## Jetty Server Dump
+
+A Jetty Server Dump is useful for debugging issues by providing details about Jetty components and their configuration, including thread pools, connectors, contexts, classloaders, servlets, and more.
+
+To enable a Jetty Server Dump after the `AppEngineWebAppContext` is started (which may occur after the first request in RPC mode), use the following system property:
+
+```xml
+<system-properties>
+  <property name="jetty.server.dumpAfterStart" value="true"/>
+</system-properties>
+```