Update documentation to clarify dependencies

Closes getpelican#460, as it documents what needs to be done to be able to use the
theme. I tested the configuration recommended in the README in a minimal
test site without translations or subsite functionality.

Also removes `i18n.patch` that was accidentally committed to the repo.

pelican-bootstrap3/README.md

@@ -26,7 +26,15 @@ Point the `THEME` variable in your `pelicanconf.py` to
 
 `JINJA_EXTENSIONS = ['jinja2.ext.i18n']`
 
-to your Pelican configuration, as this template can be translated (see below).
+to your Pelican configuration, as this template can be
+translated (see `Translations` below). You also
+need to activate a plugin that initializes the i18n jinja extension. One
+possibility is an up to date version of the
+[`i18n_subsites`](https://github.com/getpelican/pelican-plugins/tree/master/i18n_subsites)
+plugin:
+
+`PLUGIN_PATHS = ['/path/to/git/pelican-plugins']`
+`PLUGINS = ['i18n_subsites']`
 
 ## Usage