website: add pointers to the PGP signatures for releases

Peter has just pointed me to the PGP signatures of the release tarballs,
which exist since he's been doing releases.

This was not obvious to me that the releases were signed, as there was
no info about it on the website.

So, add such links on the downloads page.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/download.html

@@ -10,8 +10,10 @@ The latest stable release is <b>2013.11</b>, which can be downloaded
 here:<p>
 
 <a href="/downloads/buildroot-2013.11.tar.gz">buildroot-2013.11.tar.gz</a>
+(<a href="/downloads/buildroot-2013.11.tar.gz.sign">PGP signature</a>)
 or
-<a href="/downloads/buildroot-2013.11.tar.bz2">buildroot-2013.11.tar.bz2</a>.
+<a href="/downloads/buildroot-2013.11.tar.bz2">buildroot-2013.11.tar.bz2</a>
+(<a href="/downloads/buildroot-2013.11.tar.bz2.sign">PGP signature</a>).
 
 <p>
 <!--
@@ -19,11 +21,13 @@ The latest release candidate is <b>2013.11-rc3</b>, which can be
 downloaded here:<p>
 
 <a href="/downloads/buildroot-2013.11-rc3.tar.gz">buildroot-2013.11-rc3.tar.gz</a>
+(<a href="/downloads/buildroot-2013.11-rc3.tar.gz.sign">PGP signature</a>)
 or
-<a href="/downloads/buildroot-2013.11-rc3.tar.bz2">buildroot-2013.11-rc3.tar.bz2</a>.
+<a href="/downloads/buildroot-2013.11-rc3.tar.bz2">buildroot-2013.11-rc3.tar.bz2</a>
+(<a href="/downloads/buildroot-2013.11-rc3.tar.bz2.sign">PGP signature</a>).
 <p>
 -->
-This and earlier releases can always be downloaded from
+This and earlier releases (and their PGP signatures) can always be downloaded from
 <a href="/downloads/">http://buildroot.net/downloads/</a>.
 
 <p>