Add .phpstorm.meta.php to improve code completion

Folks, adding a `.phpstorm.meta.php` file with something like this below, would make users' experience with the package in PhpStorm a bit better when using calls like `->api('issue')`:
```php
<?php
namespace PHPSTORM_META {
    override( \Github\Client::api(0),
      map( [
        "issue" => \Github\Api\Issue::class,
        //...
    ]));
}
```
Would you be interested in adding it?
