Update hello-world GETTING_STARTED.md to match simplified exercise

[jgallagher]

Closes #265.

[IanWhitney]

Thanks!

[petertseng]

Thanks for doing this! Let's get this merged now.

For anyone looking for future directions, maybe it will be useful to relate what happens in hello-world with the rest of the track. "In future exercises, you will follow a similar process, ..." etc. Would have to think about whether that is best placed in this file or some file in the next exercise.

[jgallagher]

One thing I noticed looking at the next several exercises is that most of them don't include a src/lib.rs at all, but a few have one with all the necessary public functions defined (with unimplemented!() bodies). The latter seems a lot smoother to pick up and would reduce the need for extra process instructions compared to the former.

[petertseng]

What I see from #117 and #200 is definite agreement there should be a src/lib.rs. There wasn't agreement on whether it should be empty or contain the function signatures.

I think we can get wider agreement on signatures for the initial exercises, to help people start. On later exercises, even if we can't agree, an empty src/lib.rs would be good.