editorial pass on the README

In general, this editorial pass applies our guidelines, and there are a few details here and there:

* The names of Rails components has a space, so ActiveRecord -> Active Record, and also ActiveForm -> Active Form.
* A form object, like a model, presenter, a service, a hash table, etc. is something you write in lowercase.
* Template snippets use ERB.
* Method names without leading "#" or "::".
* Project file names with relative paths from the project root.
* Capitalization of some header.
* Completed some Ruby snippets to be well-formed (adding `end`s).
* Class names in fixed-width font.
* Blank lines before and after fenced code blocks.
Update README.md