readme updates

Author: Travis Mathis

README.md

@@ -17,7 +17,7 @@ npm install react-file-reader --save
     - fixed issue w/ uuid4 being a devDependency
   - 1.0.0
     - initial release
-    
+
 ## Props
 ### Default Props
   - fileTypes: 'image/*'
@@ -31,6 +31,9 @@ npm install react-file-reader --save
   - a function to handle the selected files from the HTML input
 
 ### Optional Props
+- elementId
+  - set a `unique` element Id for the input element
+  - if this is not set, a random UUID is generated for each element on the page.
 - base64
   - a `boolean` to convert and return the files as a base64 `string`
   - multipleFile selection will return an `array` of base64 `strings`