Complement WebUI Translation section of README.md

Hi, the pt-br l10n team is working on the translation of the website. Tico who is a member of the team is the one doing the work. He's an awesome translator, though he doesn't work with web development and programming.

When he tried to follow the steps after translating the messages, he couldn't test the work because the option to change the web page to portuguese wasn't showing up. He asked the pt-br-l10n team for help and I ended up helping him.

Turns out that one have to add the newly translated language to a list in nm2/settings.py as shown below.

diff --git a/nm2/settings.py b/nm2/settings.py
index 71a3ff60..f3d7646f 100644
--- a/nm2/settings.py
+++ b/nm2/settings.py
@@ -190,6 +190,7 @@ LANGUAGES = (
     ('es', _('Spanish')),
     ('it', _('Italian')),
     ('fr', _('French')),
+    ('pt-br', _('Portuguese')),
 )
 
 

So I think that it would be really helpful to indicate this step in the readme, so everyone from the l10n teams can test without much hassle.