Updating library/bundle install docs to use "require"

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | - 

As of composer/composer#3906, you can now simply install a library via:

```
php composer.phar require myname/my-lib
```

The latest version if chosen for you. I've replaced all mentions of modifying composer.json and running `composer update` with using the `require` command. I think the result is much simpler :).

Thanks!