Add a "contributing to flutter" section at the bottom of the community page (#5156)

Author: Hixie

src/community/index.html

@@ -8,9 +8,8 @@
     <div class="col-12">
         <h1>Welcome to the Flutter community</h1>
         <p class="community__intro__description">
-            We're open source and would love to hear from you!
             Below you'll find ways to get involved in the Flutter
-            community as well as links to
+            developer community as well as links to
             resources that can help answer your questions.
             If you're in China, read
             <a href="/community/china">Using Flutter in China</a>.
@@ -124,3 +123,16 @@ <h3>{{ case.name }}</h3>
         {% endfor -%}
     </div>
 </section>
+
+<section class="card">
+    <div class="card-body">
+        <div class="row">
+            <div class="col-lg-4">
+                <h3>Contributing to Flutter</h3>
+                <p>If you are interested in joining us in creating Flutter itself, jump over to
+                <a href="https://github.com/flutter/flutter/blob/master/CONTRIBUTING.md">our GitHub repository</a>
+                to get started!</p>
+            </div>
+        </div>
+    </div>
+</section>