README docs and minor code changes for HelloWorld app

In addition to documentation changes in the README file, I made some changes to MainActivity.java:

- Changes to comments.
- Removal of API_KEY (which is unused).
- Removal of AUTO_CONNECT and AUTO_PUBLISH (and their use in the code). These don't seem to be too instructive.
- Simplified the subscribeToSelf logic.
- Whitespace changes for consistency.