Add install instructions for CocoaPods

Explains how to install Nocilla with CocoaPods.

You also have to add `CFNetwork.framework`, otherwise compilation fails with

```
Undefined symbols for architecture i386: "_CFHTTPMessageAppendBytes", "_CFHTTPMessageCopyAllHeaderFields", "_CFHTTPMessageCopyBody", "_CFHTTPMessageCreateEmpty", "_CFHTTPMessageGetResponseStatusCode", "_CFHTTPMessageIsHeaderComplete"
```