Add 0.3.0-alpha.4 to index

Author: MajorBreakfast

index.html

@@ -10,6 +10,9 @@ <h1 align="center">
     </h1>
 
     <main id="doc-links">
+        <a href="https://rust-lang-nursery.github.io/futures-api-docs/0.3.0-alpha.4/futures/">
+          <span>0.3.0-alpha.4</span>
+        </a>
         <a href="https://rust-lang-nursery.github.io/futures-api-docs/0.3.0-alpha.3/futures/">
           <span>0.3.0-alpha.3</span>
         </a>