Update README.md with editing mode documentation

README.md

@@ -15,6 +15,67 @@ The webapp embeds a YouTube video, and overlays an OpenStreetMap map view over i
 
 Note that timing & location data must be entered manually with this webapp; this data is not extracted automatically from the video.
 
+## Editing mode
+This mode allows users to add and remove timing points to refine the syncing of map and video.
+- Click the **EDITING** button in the upper-right corner of the screen to open editing mode.
+
+![Screenshotshowing position of editing button]()
+
+### Timing points map
+The **timing points map** appears on the right side of the **editing** mode screen. It displays the track path and train position.
+
+![Screenshot highlighting map]()
+
+- The position of the train as referenced in the video is indicated by the **blue pin.**
+- The track path as defined by the OSM XML file appears as a **purple line.**
+- Timing points are indicated by **green circles.**
+
+#### Changing the map view
+- Zoom in and out with:
+  - Buttons in the upper-left corner of the map.
+  - Mouse wheel.
+  - '-' and '+' keys.
+- While the video is paused, pan with:
+  - Mouse click and drag.
+  - Arrow keys. 
+
+#### Edit track geomoetry button. 
+- I don't think this currently does anything.
+
+### Add timing point
+Adding timing points refines the syncing of the video and map.
+
+![Screenshot highlighting add timing point button]()
+
+- Click **add timing point** button to add a timing point at the location of the crosshairs in the **timing points map.**
+  - t = video time elapsed (in seconds).
+  - d = distance traveled along track path (in meters).
+
+- **Tips**
+  1. Look for landmarks that are clearly visible in both video and map. *Examples: road crossings, bridges, buildings, stations.*
+  2. Pause the video when the train crosses these points.
+  3. Drag the crosshairs on the **timing points map** to the appropriate location.
+  4. Place a timing point by clicking the **add timing point** button.
+
+### Timing points list
+Displays all current timing points in order by video time elapsed and distance traveled along track path. Timing points appear as green circles on the **timing points map.**
+- Click ... and select 'delete' to remove a timing point.
+
+![Screenshot highlighting timing points list}()
+
+
+### Import OSM XML
+- Imports XML path map from OpenStreetMaps.
+- Need a better explanation of how this works. 
+
+![Screenshot highlighting Import OSM XML button]()
+
+### Reverse path
+- Reverses the map path to run end -. start.
+- Why would a user do this?
+
+![Screenshot highlighting Reverse path button]()
+
 ---
 
 ## License