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

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.

Open the repository README and find the installation code block containing both npm and yarn commands. Separate the commands into independent code blocks so each can be copied and pasted safely; done means the README presents npm and yarn alternatives without combining them in one block.

Written by the indexing model from the issue text.