Add alt attribute to img element in HTML files

index.html

@@ -136,7 +136,7 @@ layout: home_orekit
 
     <p>
       <a href="https://bestpractices.coreinfrastructure.org/projects/2726">
-        <img src="https://bestpractices.coreinfrastructure.org/projects/2726/badge"/>
+        <img src="https://bestpractices.coreinfrastructure.org/projects/2726/badge" alt="Core Infrastructure Initiative (CII) badge"/>
       </a>
     </p>
     <p>We do our best to provide you with a quality code:</p>