added getMultiDayStepCounts method docs

README.md

@@ -18,6 +18,7 @@ A React Native bridge module for interacting with [Apple HealthKit] data.
       * [initHealthKit](#inithealthkit)
       * [getStepCountForToday](#getstepcountfortoday)
       * [getStepCountForDay](#getstepcountforday)
+      * [getMultiDayStepCounts](#getmultidaystepcounts)
       * [getLatestWeight](#getlatestweight)
       * [saveWeight](#saveweight)
       * [getLatestHeight](#getlatestheight)
@@ -204,6 +205,34 @@ AppleHealthKit.getStepCountForDay(options, (err, steps) => {
 
 ___
 
+#### **`getMultiDayStepCounts`**
+get the total steps per day over a specified date range. 
+
+`getMultiDayStepCounts` accepts an options object containing *`startDate: ISO8601Timestamp`*, and optional *`endDate: ISO8601Timestamp`* fields. if `endDate` is not provided it will default to the current time
+```javascript
+let options = {
+    startDate: (new Date(2016,5,1)).toISOString()  // required
+    endDate:   (new Date()).toISOString()          // optional; default now
+};
+```
+The function will be called with an array of elements `res` containing date and step count information
+```javascript
+ AppleHealthKit.getMultiDayStepCounts(options, (err: Object, res: Array<Array<string|number>>) => {
+    if(this._handleHealthKitError(err, 'getMultiDayStepCounts')){
+        return;
+    }
+    // 'res' is array of [ISOTimestamp: string, stepCount: number] arrays
+    // sorted ascending from startDate through endDate (default now)
+    for(let i=0; i<res.length; ++i){
+        let elem = res[i];
+        // elem[0] is ISOTimestamp : string
+        // elem[1] is stepCount    : number
+    }
+});
+```
+
+___
+
 #### **`getLatestWeight`**
 get the most recent weight value
 ```javascript