Documentation basic usage missing a get_env example

[cglacet]

I've spent the last 10 minutes looking for how to add a global to jinja2 environment (get_env, code).

For example, adding the zip function:

env = aiohttp_jinja2.get_env(app)
env.globals.update(zip=zip)

It appears to me as basic and that would be nice to have it referenced, but maybe I'm wrong ^^

[asvetlov]

Feel free to make a pull request, please

[asvetlov]

The change is published in 1.1.2 release.