Add the error code for missing user agent

This requirement is somewhat *hidden* in the documentation.
Besides that still residing problem, the pull allows to:
- google the problem
- adds what the ```Status 403: Forbidden``` could be caused by

The problem for e.g. PHP's cURL is, that the [user agent is not set by default](https://github.com/ax3l/github_status_proxy/commit/4d3b205b467fbc260eb2735e80b09e31b504b9e7).

Example:
```php
$ch = curl_init();
...
curl_setopt($ch, CURLOPT_USERAGENT, "Awesome-Octocat-App");
...
```