Updated TextInput prop doc

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I am a web developer, who recently started coding for mobile apps using react-native. I was trying to use the `inlineImageLeft` props of TextInput, but I found that it's docs weren't sufficient. So a PR for it.

No code change. Updated the docs under website folder. Screenshot for the change below.

![screen shot 2017-08-30 at 4 39 32 pm](https://user-images.githubusercontent.com/6011865/29869747-e73d9dde-8da1-11e7-912a-16e3115b8296.png)
Closes https://github.com/facebook/react-native/pull/15708

Differential Revision: D5738795

Pulled By: hramos

fbshipit-source-id: b8b6cbac5c50abd4d8a6ef8089dc9d92bc0b7f6f

Libraries/Components/TextInput/TextInput.js

@@ -513,6 +513,13 @@ const TextInput = createReactClass({
 
     /**
      * If defined, the provided image resource will be rendered on the left.
+     * The image resource must be inside `/android/app/src/main/res/drawable` and referenced
+     * like
+     * ```
+     * <TextInput
+     *  inlineImageLeft='search_icon'
+     * />
+     * ```
      * @platform android
      */
     inlineImageLeft: PropTypes.string,