update docs and comments

README.md

@@ -16,6 +16,34 @@ var cell = S2.S2Cell.FromLatLng(latlng, level);
 cell.getNeighbors();  // [ cellLeft, cellDown, cellRight, cellUp ]
 
 cell.getLatLng();     // { lat: 40.2574448, lng: -111.7089464 }
+
+var key = cell.toHilbertQuadkey();
+```
+
+Previous and Next
+-----------------
+
+You can get the previous and next S2CellId from any given Key:
+
+1. Convert from Lat/Lng to Key (Face and Hilbert Curve Quadtree)
+2. Get the Previous or Next Key
+3. Convert the Key to an Id (uint64 string)
+
+```
+var key = S2.latLngToKey(40.2574448, -111.7089464);
+var id = S2.toId(key);
+
+var nextKey = S2.nextKey(key);
+var nextId = S2.toId(nextKey);
+
+var prevKey = S2.prevKey(key);
+var prevId = S2.toId(prevKey);
+
+// See it
+console.log(prevKey);
+console.log(key);
+console.log(nextKey);
+console.log(nextId);
 ```
 
 convert Cell Id to Quadkey

src/s2geometry.js

@@ -375,6 +375,8 @@ S2.latLngToKey = S2.latLngToQuadkey = function (lat, lng, level) {
   //
   // S2.idToLatLng(id)
   // S2.keyToLatLng(key)
+  // S2.nextFace(key)     // prevent wrapping on nextKey
+  // S2.prevFace(key)     // prevent wrapping on prevKey
   //
   // .toKeyArray(id)  // face,quadtree
   // .toKey(id)       // hilbert
@@ -382,6 +384,7 @@ S2.latLngToKey = S2.latLngToQuadkey = function (lat, lng, level) {
   // .toId(key)       // uint64 (as string)
   // .toLong(key)     // long.js
   // .toLatLng(id)    // object? or array?, or string (with comma)?
+  //
   // maybe S2.HQ.x, S2.GPS.x, S2.CI.x?
   return S2.S2Cell.FromLatLng({ lat: lat, lng: lng }, level).toHilbertQuadkey();
 };