Add note about gnutls and libgpg-error in installation instructions

doc/index.html

@@ -110,7 +110,8 @@ server.listen(7000, "localhost");</pre>
         tested on <b>Linux</b>, <b>Macintosh</b>, and <b>FreeBSD</b>. The
         build system requires Python 2.4 or better.  V8, on which Node is
         built, supports only IA-32 and ARM processors. V8 is included in the
-        Node distribution. There are no other dependencies.
+        Node distribution. To use TLS, GnuTLS and libgpg-error are required.
+        There are no other dependencies.
       </p>
 
       <pre class="sh_none">