[release notes] Add a 5.x header

Closes #3130

doc/releases.html

@@ -14,7 +14,8 @@
     <li><a href="https://github.com/codemirror/codemirror">Code</a>
   </ul>
   <ul>
-    <li><a class=active data-default="true" href="#v4">Version 4.x</a>
+    <li><a class=active data-default="true" href="#v5">Version 5.x</a>
+    <li><a href="#v4">Version 4.x</a>
     <li><a href="#v3">Version 3.x</a>
     <li><a href="#v2">Version 2.x</a>
     <li><a href="#v1">Version 0.x</a>
@@ -25,9 +26,9 @@
 
 <h2>Release notes and version history</h2>
 
-<section id=v4 class=first>
+<section id=v5 class=first>
 
-  <h2>Version 4.x</h2>
+  <h2>Version 5.x</h2>
 
   <p class="rel">20-02-2015: <a href="http://codemirror.net/codemirror-5.0.zip">Version 5.0</a>:</p>
 
@@ -39,6 +40,12 @@
     <li>Full <a href="https://github.com/codemirror/CodeMirror/compare/4.13.0...5.0.0">list of patches</a>.</li>
   </ul>
 
+</section>
+
+<section id=v4 class=first>
+
+  <h2>Version 4.x</h2>
+
   <p class="rel">20-02-2015: <a href="http://codemirror.net/codemirror-4.13.zip">Version 4.13</a>:</p>
 
   <ul class="rel-note">
@@ -1033,12 +1040,12 @@
   parser. And, as usual, add workarounds for various newly discovered
   browser incompatibilities.</p>
 
-  <p class="rel"><em>31-08-2009</em>: <a href="http://codemirror.net/codemirror-0.63.zip">Version 0.63</a>:</p>
+  <p class="rel">31-08-2009: <a href="http://codemirror.net/codemirror-0.63.zip">Version 0.63</a>:</p>
   <p class="rel-note"> Overhaul of paste-handling (less fragile), fixes for several
   serious IE8 issues (cursor jumping, end-of-document bugs) and a number
   of small problems.</p>
 
-  <p class="rel"><em>30-05-2009</em>: <a href="http://codemirror.net/codemirror-0.62.zip">Version 0.62</a>:</p>
+  <p class="rel">30-05-2009: <a href="http://codemirror.net/codemirror-0.62.zip">Version 0.62</a>:</p>
   <p class="rel-note">Introduces <a href="contrib/python/index.html">Python</a>
   and <a href="contrib/lua/index.html">Lua</a> parsers. Add
   <code>setParser</code> (on-the-fly mode changing) and