Getting Started guide: update for new versions of Mercurial

Newer versions of Mercurial require the configuration
of Certification Authorities.

R=rsc
CC=golang-dev
https://golang.org/cl/4128052

doc/install.html

@@ -110,6 +110,21 @@ package repository will most likely be old and broken.)
 If that fails, try installing manually from the <a href="http://mercurial.selenic.com/wiki/Download">Mercurial Download</a> page.</p>
 </p>
 
+<p>
+Mercurial versions 1.7.x and up require the configuration of
+<a href="http://mercurial.selenic.com/wiki/CACertificates">Certification Authorities</a>
+(CAs). Error messages of the form:
+</p>
+<pre>
+warning: go.googlecode.com certificate with fingerprint b1:af: ... bc not verified (check hostfingerprints or web.cacerts config setting)
+</pre>
+<p>
+when using Mercurial indicate that the CAs are missing.
+Check your Mercurial version (<code>hg --version</code>) and
+<a href="http://mercurial.selenic.com/wiki/CACertificates#Configuration_of_HTTPS_certificate_authorities">configure the CAs</a>
+if necessary.
+</p>
+
 <h2 id="fetch">Fetch the repository</h2>
 
 <p>