added saveBMI method docs

README.md

@@ -18,6 +18,7 @@ A React Native bridge module for interacting with [Apple HealthKit] data.
       * [getLatestHeight](#getlatestheight)
       * [saveHeight](#saveheight)
       * [getLatestBmi](#getlatestbmi)
+      * [saveBmi](#savebmi)
       * [getLatestBodyFatPercentage](#getlatestbodyfatpercentage)
       * [getLatestLeanBodyMass](#getlatestleanbodymass)
   * [Examples](#examples)
@@ -266,6 +267,24 @@ AppleHealthKit.getLatestBmi(null, (err: string, bmi: Object) => {
 
 ___
 
+#### **`saveBmi`**
+save a numeric BMI value to HealthKit
+
+`saveBmi` accepts an options object containing a numeric BMI value:
+```javascript
+let options = {value: 27.2}
+```
+```javascript
+AppleHealthKit.saveBmi(options, (err, res) => {
+    if(this._handleHealthKitError(err, 'saveBmi')){
+        return;
+    }
+    // BMI successfully saved
+});
+```
+
+___
+
 #### **`getLatestBodyFatPercentage`**
 get the most recent body fat percentage. the percentage value is a number between 0 and 100
 ```javascript