Move materials to docs/img folder

README.md

 <p align="center">
-<img src="https://microsoft.github.io/nni/docs/img/nni_logo.png" width="300"/>
+<img src="https://github.com/Microsoft/nni/blob/master/docs/img/nni_logo.png" width="300"/>
 </p>
 
 -----------
@@ -18,22 +18,22 @@ The tool dispatches and runs trial jobs generated by tuning algorithms to search
 
 ### **NNI [v0.5.1](https://github.com/Microsoft/nni/releases) has been released!**
 <p align="center">
-  <a href="#nni-v05-has-been-released"><img src="https://microsoft.github.io/nni/docs/img/overview.svg" /></a>
+  <a href="#nni-v05-has-been-released"><img src="https://github.com/Microsoft/nni/blob/master/docs/img/overview.svg" /></a>
 </p>
 <table>
   <tbody>
     <tr align="center" valign="bottom">
       <td>
         <b>Supported Frameworks</b>
-        <img src="https://user-images.githubusercontent.com/44491713/51381727-e3d0f780-1b4f-11e9-96ab-d26b9198ba65.png"/>
+        <img src="https://github.com/Microsoft/nni/blob/master/docs/img/bar.png"/>
       </td>
       <td>
         <b>Tuning Algorithms</b>
-        <img src="https://user-images.githubusercontent.com/44491713/51381727-e3d0f780-1b4f-11e9-96ab-d26b9198ba65.png"/>
+        <img src="https://github.com/Microsoft/nni/blob/master/docs/img/bar.png"/>
       </td>
       <td>
         <b>Training Services</b>
-        <img src="https://user-images.githubusercontent.com/44491713/51381727-e3d0f780-1b4f-11e9-96ab-d26b9198ba65.png"/>
+        <img src="https://github.com/Microsoft/nni/blob/master/docs/img/bar.png"/>
       </td>
     </tr>
     <tr/>