Update README.md

Updated info for the Podfile because of deprecation warning:

```
[!] The use of implicit sources has been deprecated. To continue using all of the sources currently on your machine, add the following to the top of your Podfile:

    source 'https://github.com/CocoaPods/Specs.git'
```
Merge pull request #1 from jaltek/patch-1

Update info for the Podfile