docs: add Playback from Segments docs

Author: nklhtv

doc/segments-playback.md

@@ -0,0 +1,41 @@
+## Segments Playback
+
+The player supports a feature called 'Playback from Segments'. This lets you to click on a segment in your app (i.e. a paragraph) to begin playback from that segment. If the segment is already playing then it will be paused instead.
+
+The segments playback in the iOS Player is based on the [Segments Playback in the Web Player](https://github.com/beyondwords-io/player/blob/main/doc/segments-playback.md). The iOS SDK cannot automatically identify segments, instead you should be able to manually link each text segment with its beyonwordas marker.
+
+You can find an example of how segment playback can be integrated in your app in [PlaybackFromSegmentsViewController.swift](../Example/Example/PlaybackFromSegmentsViewController.swift)
+
+## How it works
+
+To highlight the current segment you have to listen for the `CurrentSegmentUpdated` event, then find the correspondig UI element to the `currentSegment` and apply the desired styling to it.
+
+```swift
+extension PlaybackFromSegmentsViewController : PlayerDelegate {
+    public func player(_ playerView: PlayerView, onEvent event: PlayerEvent, settings: PlayerSettings) {
+        if (self.playerView !== playerView) { return }
+        
+        if (event.type == "CurrentSegmentUpdated") {
+            let text = settings.currentSegment?.marker.flatMap { self.segments[$0] }
+            for view in self.contentView.arrangedSubviews {
+                guard let label = view as? UILabel else { continue }
+                if (label.text == text) {
+                    label.backgroundColor = .lightGray
+                } else {
+                    label.backgroundColor = .clear
+                }
+            }
+        }
+    }
+}
+```
+
+To change the current time of the player when a segment is clicked you have to set `UITapGestureRecognizer` and call `setCurrentSegment` with the correspondig marker.
+
+```swift
+@objc private func segmentTapped(_ gesture: UITapGestureRecognizer) {
+    guard let label = gesture.view as? UILabel else { return }
+    guard let marker = self.segments.first(where: { $0.value == label.text })?.key else { return }
+    playerView.setCurrentSegment(segmentMarker: marker)
+}
+```