Docs: add note on pre- & post- being available for run-scripts as well.

After [learning something](http://sudodoki.name/til-prepost-are-for-everybody/) just thought it might be useful to put in documentation which might help some people make their flow easier and stop putting too much '&&' in their custom scripts.

This PR adds a note about the fact, that  if you script section looks like this:
```
{
  "scripts": {
    "premyscript": "echo \"Pre stuff\"",
    "myscript": "echo \"LOL MY SCRIPT\" ",
    "postmyscript":"echo \"Post stuff\""
  }
}
```
invoking `npm run myscript` would run `premyscript`, then `myscript` and `postmyscript` in the end.