Readme security improvement: Split up `npm` and `yarn` code blocks

[plocket]

Currently, the README has

$ npm i better-ajv-errors
$ # Or
$ yarn add better-ajv-errors

One problem with this is that it doesn't allow the developer to just hit 'copy' and paste it into their terminal. npm has problems with people "typo squatting", so copy/pasting is a safer and more secure option than re-typing by hand. Separate code blocks for each command would let a developer stay safer.

[plocket]

I made a PR, but can't follow through because the form asks for PI I'm not willing to give. It's pretty straightforward, though, if someone else wants to do it.