IACO-527: GitHub documentation

It would probably be best to read the documentation instead of looking at the markdown, so feel free to open up the following links:
https://github.com/yuval-netanel/inapp-ios/tree/IACO-527-github-documentation
https://github.com/yuval-netanel/inapp-ios/blob/IACO-527-github-documentation/doc/integration.md

As a semi-outsider, you should definitely give this a glance, @rauchy.
-I think we should recomend to do this in the AppDelegate.didFinishLaunchingWithOptions:
@implementation AppDelegate
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
  [KIAContext setApiKey:@"test_29f612e8-1576-423f-80a8-679f354e4c89"];
  return YES;
}

- Also For testing user can use test_29f612e8-1576-423f-80a8-679f354e4c89 for now (there no need to wait for key)
- @tomeralon We need to specify how the user can get key. for now Maybe we should open an official app@klarna.com email which will lead to you and me