update README

README.md

@@ -197,6 +197,14 @@ maxJobs = 6 : number
 
 The maximum number of jobs to be processing at once.
 
+### .unloadPriorityCallback
+
+```js
+unloadPriorityCallback = null : ( item ) => Number
+```
+
+Function to derive the unload priority of the given item. Higher priority values get unloaded first.
+
 ## LRUCache
 
 Utility class for the TilesRenderer to keep track of currently used items so rendered items will not be unloaded.
@@ -225,6 +233,14 @@ unloadPercent = 0.05 : number
 
 The maximum percentage of [minSize](#minSize) to unload during a given frame.
 
+### .priorityCallback
+
+```js
+priorityCallback = null : ( item ) => Number
+```
+
+Function to derive the job priority of the given item. Higher priority values get processed first.
+
 # LICENSE
 
 The software is available under the [Apache V2.0 license](../LICENSE.txt).