update document about options update

docs/developers/updates.md

@@ -1,6 +1,6 @@
 # Updating Charts
 
-It's pretty common to want to update charts after they've been created. When the chart data is changed, Chart.js will animate to the new data values. 
+It's pretty common to want to update charts after they've been created. When the chart data or options are changed, Chart.js will animate to the new data values and options.
 
 ## Adding or Removing Data
 
@@ -26,6 +26,46 @@ function removeData(chart) {
 }
 ```
 
+## Opdating Options
+
+To update the options, mutating the options propery inplace or passing in a new options object are supported.
+
+- If the options are mutated inplace, other options properties would be preserved.
+- If created as a new object, it would be like creating a new chart with the options - old options would be discarded.
+
+```javascript
+function updateConfigByMutating(chart) {
+    chart.options.title.text = 'new title';
+}
+```
+
+```javascript
+function updateConfigAsNewObject(chart) {
+    chart.options = {
+        responsive: true,
+        title:{
+            display:true,
+            text: 'Chart.js'
+        },
+        scales: {
+            xAxes: [{
+                display: true
+            }],
+            yAxes: [{
+                display: true
+            }]
+        }
+    }
+}
+```
+
+Scales can be updated separately without changing other options.
+It is recommended to update scales by creating a new object rather than mutating the properties to be changed, unless you are confident all scales options would update as you wish - especially when upating type of the scale.
+
+Variables referencing the scale of the chart would be lost after updating with a new id or the changed type.
+
+Code sample for updating options can be found in [toggle-scale-type.html](../../samples/scales/toggle-scale-type.html).
+
 ## Preventing Animations
 
 Sometimes when a chart updates, you may not want an animation. To achieve this you can call `update` with a duration of `0`. This will render the chart synchronously and without an animation.