Initial setup for guides

Summary:
- Create `Guides/` top-level dir
- Move `README` "creating first list" contents into `Guides/Getting Started.md`
- Add initial `Guides/Migration.md` (ref #221)
- Fix #245

See rendered guides at:

- https://github.com/Instagram/IGListKit/blob/guides-setup/Guides/Getting%20Started.md
- https://github.com/Instagram/IGListKit/blob/guides-setup/Guides/Migration.md
Closes https://github.com/Instagram/IGListKit/pull/252

Differential Revision: D4228332

Pulled By: jessesquires

fbshipit-source-id: f26c2c1ee3cfefb40be1f99674702f95394d4aaf

docs/Classes/IGListGridCollectionViewLayout.html

@@ -28,6 +28,17 @@
     <div class="content-wrapper">
       <nav class="sidebar">
         <ul class="nav-groups">
+          <li class="nav-group-name">
+            <a href="../Guides.html">Guides</a>
+            <ul class="nav-group-tasks">
+              <li class="nav-group-task">
+                <a href="../getting-started.html">Getting Started</a>
+              </li>
+              <li class="nav-group-task">
+                <a href="../migration.html">Migration</a>
+              </li>
+            </ul>
+          </li>
           <li class="nav-group-name">
             <a href="../Categories.html">Categories</a>
             <ul class="nav-group-tasks">
@@ -301,7 +312,7 @@ When the size is zero (the default), then the layout will query the collection v
           </section>
         </section>
         <section id="footer">
-          <p>&copy; 2016 <a class="link" href="https://twitter.com/fbOpenSource" target="_blank" rel="external">Instagram</a>. All rights reserved. (Last updated: 2016-11-21)</p>
+          <p>&copy; 2016 <a class="link" href="https://twitter.com/fbOpenSource" target="_blank" rel="external">Instagram</a>. All rights reserved. (Last updated: 2016-11-23)</p>
           <p>Generated by <a class="link" href="https://github.com/realm/jazzy" target="_blank" rel="external">jazzy ♪♫ v0.7.2</a>, a <a class="link" href="http://realm.io" target="_blank" rel="external">Realm</a> project.</p>
         </section>
       </article>