docs/website: add paragraph about the IRC channel restrictions

Due to excessive spamming on Freenode, the IRC channel is currently
restricted to registered users. Since this may not be obvious for
people not familiar with IRC, let's mention it on our website, with a
link to the relevant documentation.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

docs/website/support.html

@@ -24,6 +24,12 @@
 		    use the <a href="http://webchat.freenode.net">Freenode web interface</a>.
 		    When asking for help on IRC, share relevant logs or pieces of code using a code
 		    sharing website.</p>
+
+		  <p>Note that due to excessive spamming on Freenode, the channel can only be
+		    joined if you are a registered user with
+		    <a href="https://freenode.net/kb/answer/registration">Freenode nickserv</a>
+		    service. Follow the instructions to register as a user with a password,
+		    and then join the <code>#buildroot</code> channel.</p>
 		</div>
 	      </div>
 	    </div>