Updates documentation to be more clear

It took me a bit to realize that `has_merit` needs to be added to the model, since it's not in the installation instructions.

I also changed "Yearling" to "year-member", since it demonstrates the actual convention for naming badges.

Also one typo.
Thanks for your PR! Though actually this line should be adding the `has_merit` call into the model. If for whatever reason it doesn't work this way in your app, let's update the line to say something like: `3. Run `rails g merit MODEL_NAME` (e.g. `user`). Adds a `has_merit` call to the model.`.