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.
I figured that this generator was supposed to add the call. `rails g merit user` creates the migration, but does not add the `has_merit` call to the model on my app. I just realized that my `user` model is in a subdirectory of `models`, and that is what is preventing the insertion from working. 

Like you say this generator might not work for any number of reasons (it is software after all!), so adding the documentation will let developers know to manually insert the content if they end up in a situation like mine.