Add readme with supported flags for log4j 1 appender instrumentation (#14704)

Author: laurit

instrumentation/log4j/log4j-appender-1.2/javaagent/README.md

@@ -0,0 +1,9 @@
+# Settings for the Log4j Appender instrumentation
+
+| System property                                                                   | Type    | Default | Description                                                                                                                                   |
+|-----------------------------------------------------------------------------------|---------|---------|-----------------------------------------------------------------------------------------------------------------------------------------------|
+| `otel.instrumentation.log4j-appender.experimental-log-attributes`                 | Boolean | `false` | Enable the capture of experimental log attributes `thread.name` and `thread.id`.                                                              |
+| `otel.instrumentation.log4j-appender.experimental.capture-code-attributes`        | Boolean | `false` | Enable the capture of [source code attributes]. Note that capturing source code attributes at logging sites might add a performance overhead. |
+| `otel.instrumentation.log4j-appender.experimental.capture-mdc-attributes`         | String  |         | Comma separated list of context data attributes to capture. Use the wildcard character `*` to capture all attributes.                         |
+
+[source code attributes]: https://github.com/open-telemetry/semantic-conventions/blob/main/docs/general/attributes.md#source-code-attributes