[ios] add user interaction guide (#7937)

* added gesture recognizer guide and gifs

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+# User Interactions
+
+The Mapbox iOS SDK provides a set of built-in gesture recognizers. You can customize or supplement these gestures according to your use case. You see what gesture recognizers are on your `MGLMapView` by accessing the `gestureRecognizers` property on your map.
+
+## Configuring user interaction
+
+Several properties on an `MGLMapView` provide ways to enable or disable a set of gesture recognizers. Boolean values are set to `YES` by default.
+
+- `zoomEnabled` - Allows the user to zoom in or out by pinching two fingers, double-tapping, tapping with two fingers, or double-tapping then dragging vertically. Accepts Boolean values.
+- `scrollEnabled` - Allows the user to scroll by dragging or swiping one finger. Accepts Boolean values.
+- `rotateEnabled` - Allows the user to rotate by moving two fingers in a circular motion. Accepts Boolean values.
+- `pitchEnabled` - Allows the user to tilt the map by vertically dragging two fingers. Accepts Boolean values.
+- `decelerationRate` - Determines the rate of deceleration after the user lifts their finger. You can set the value using the  `MGLMapViewDecelerationRateNormal`, `MGLMapViewDecelerationRateFast`, or `MGLMapViewDecelerationRateImmediate` constants.
+
+## Individual gestures
+
+|Gesture | Description | Related Property |
+|:-------:|----------------| -----------|
+|Pinch    | Zooms in or out on the map's anchor point | `zoomEnabled` |
+|Rotation | Changes the MGLMapView direction based on the user rotating two fingers in a circular motion | `rotateEnabled` |
+|Single tap | Selects/deselects the annotation that you tap. | |
+|Double tap | Zooms in on the map's anchor point | `zoomEnabled` |
+|Two-finger tap | Zooms out with the map's anchor point centered | `zoomEnabled` |
+|Pan | Scrolls across mapView (_note: if_ `MGLUserTrackingModeFollow` _is being used, it will be disabled once the user pans_)| `scrollEnabled` |
+|Two-finger drag | Adjusts the pitch of the `MGLMapView` | `pitchEnabled` |
+|One-finger zoom | Tap twice; on second tap, hold your finger on the map and pan up to zoom in, or down to zoom out | `zoomEnabled`|
+
+![quick zoom](img/user-interaction/quickzoom.gif) ![rotation](img/user-interaction/RotateSydney.gif)
+
+## Adding custom gesture recognizers
+
+You can add `UIGestureRecognizers` to your map programmatically or via storyboard. Adding custom responses to gesture recognizers can enhance your user's experience, but try to use standard gestures where possible.
+
+The gesture recognizers that you add will take priority over the built-in gesture recognizer. You can also set up your own gesture recognizer to work simultaneously with built-in gesture recognizers by using `-gestureRecognizer:shouldRecognizeSimultaneouslyWithGestureRecognizer:`, allowing you to enhance already existing gesture recognizers.
+
+You can also add gesture recognizers that are only called when the default gesture recognizer fails (and vice versa), such as when a user taps on a part of the map that is not an annotation. The documentation for [MGLMapView](Classes/MGLMapView.html) includes an example of how to create a fallback gesture recognizer.
+
+If you would like to disable a specific set of gesture recognizers, such as zoom, you can set the Boolean value for the appropriate property to `NO`. You can then add your own gesture recognizers to perform those actions.