Add pip installation command

Closes #786

download.html.in

@@ -33,6 +33,10 @@
 <code class="commands">sudo wget @PROGRAM_URL@ -O /usr/local/bin/youtube-dl<br/>sudo chmod a+x /usr/local/bin/youtube-dl</code>
 </p>
 
+<p>You can also use pip:
+<code class="commands">sudo pip install --upgrade youtube_dl</code>
+</p>
+
 <p>To check the signature, type:
 <code class="commands">sudo wget @PROGRAM_URL@.sig -O youtube-dl.sig<br/>gpg --verify youtube-dl.sig /usr/local/bin/youtube-dl<br/>rm youtube-dl.sig</code>
 </p>