Add doc about launch and test files.

While this kind of instructional document fits better to [the tutorial page](http://wiki.ros.org/rtmros_nextage/Tutorials), I guess in the not-so-far future we'd like to migrate our document location to the repository ([discussion](https://github.com/tork-a/rtmros_nextage/issues/12)). So except for the particular subjects that are with immediate need, I think I'll keep adding documents to the repo like this.