Add an example for showing/dismissing the FBTweakViewController manually (#114)

* Add an example for showing/dismissing the FBTweakViewController manually

* Clarify README instructions for manually presenting FBTweakViewController to reference an existing view controller rather than reaching out to a global one.

* Fix typo

Author: warpling

README.md

@@ -91,7 +91,19 @@ To configure your tweaks, you need a way to show the configuration UI. There's t
 ```
 
  - You can present a `FBTweakViewController` from anywhere in your app. Be sure to restrict the activation UI to debug builds!
- 
+ ```objective-c
+ - (void) showTweaks {
+    FBTweakViewController *tweakVC = [[FBTweakViewController alloc] initWithStore:[FBTweakStore sharedInstance]];
+    tweakVC.tweaksDelegate = self;
+    // Assuming this is in the app delegate
+    [self.window.rootViewController presentViewController:tweakVC animated:YES completion:nil];
+}
+
+- (void) tweakViewControllerPressedDone:(FBTweakViewController *)tweakViewController {
+    [tweakViewController dismissViewControllerAnimated:YES completion:NULL];
+}
+```
+
 #### Tweaks UI Dismiss Notification
 
 Alternatively, when the Tweaks UI is dismissed, you can register your notification center to listen to `FBTweakShakeViewControllerDidDismissNotification`, which can be used after importing `FBTweakViewController.h`