Add a prettierignore file

[jameslittle230]

From the Prettier docs:

It’s recommended to always make sure that prettier --write . only formats what you want in your project. Use a .prettierignore file to ignore things that should not be formatted.

prettier --write . does not do the right thing currently. There should be a prettierignore file so that Prettier only operates on the /js directory.

[jameslittle230]

cc @healeycodes

[healeycodes]

I'll take a look 😄