need to improve security document in rcljava android examples.

[pengzhouhu]

@jacobperron hi jacob, i take the issue here:https://github.com/ros2-java/ros2_java/issues/197, my real mind is that when we need add security model through rcljava android examples, like set some security args, we miss a document to discribe how people can do it.

the example miss this case, do we need to improve it ?

[jacobperron]

If you are able to contribute documentation for how to enable security for rcljava on Android, I'm happy to review a pull request :)

[pengzhouhu]

If you are able to contribute documentation for how to enable security for rcljava on Android, I'm happy to review a pull request :)

no problem，i will take it as my homework and push as soon.