[wwwdocs] Describe how to validate wwwdocs changes

[Gerald Pfeifer]

This is triggered by a report from Martin who rightfully pointed 
out that it's not straightforward to validate wwwdocs changes before 
committing a change.

Martin, what do you think?  Would that have avoided the challenges
your ran into?  Anything to better clarify or otherwise improve?

Gerald

Index: about.html
===================================================================
RCS file: /cvs/gcc/wwwdocs/htdocs/about.html,v
retrieving revision 1.28
diff -u -r1.28 about.html
--- about.html  6 May 2018 16:19:24 -0000       1.28
+++ about.html  14 May 2018 04:11:25 -0000
+<h3>Validating a change</h3>
+
+<p>To validate any changes, you can use the <a
+href="https://validator.w3.org";>W3 Validator</a>. Just replace the
+&lt;html&gt; tag at the beginning of the document with the following
+snippet and use the "Validate by File Upload" functionality.</p>
+
+<blockquote><pre>
+&lt;?xml version="1.0" encoding="utf-8"?&gt;
+&lt;!DOCTYPE html
+  PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
+  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"&gt;
+&lt;html xmlns="http://www.w3.org/1999/xhtml"; xml:lang="en" lang="en"&gt;
+</pre></blockquote>
+
 <h3>Checking in a change</h3>
 
 <p>We recommend you list files explicitly to avoid accidental checkins