doc: add spinner style demo to readme (#9)

Author: kattouf

.github/activity_style_dots.gif

@@ -2,7 +2,7 @@ Output out.gif
 
 Set Shell "zsh"
 Set FontSize 32
-Set Width 620
+Set Width 660
 Set Height 300
 Set TypingSpeed 75ms
 

.github/activity_style_kitt.gif

@@ -43,9 +43,24 @@ long-running-command | progressline --activity-style snake
 
 Available styles:
 
-| dots (Default) | snake | [kitt](https://en.wikipedia.org/wiki/KITT) |
-|:--:|:--:|:--:|
-| ![](./.github/activity_style_dots.gif) | ![](./.github/activity_style_snake.gif) | ![](./.github/activity_style_kitt.gif) |
+<table>
+  <tr>
+    <td align="center">dots (Default)</td>
+    <td align="center">snake</td>
+    <td align="center"><a href="https://en.wikipedia.org/wiki/KITT">kitt</a></td>
+  </tr>
+  <tr>
+    <td><img src="./.github/activity_style_dots.gif" alt="dots" width="300"></td>
+    <td><img src="./.github/activity_style_snake.gif" alt="snake" width="300"></td>
+    <td><img src="./.github/activity_style_kitt.gif" alt="kitt" width="300"></td>
+  </tr>
+  <tr>
+    <td align="center">spinner</td>
+  </tr>
+  <tr>
+    <td><img src="./.github/activity_style_spinner.gif" alt="spinner" width="300"></td>
+  </tr>
+</table>
 
 ### Replace log output with custom text