Added doc on template ordering

[ritwickdsouza]

References #62
Please let me know if any changes needs to be made. :)

[coveralls]

Coverage remained the same at 100.0% when pulling ad4f9e3 on ritwickdsouza:ordering-templates into 5fc47f9 on openwisp:master.

[nemesifier]

Good! Please improve formatting as suggested. Test your changes locally with by doing:

cd docs
make html

And check the generated HTML looks good.

[nemesifier]

I would add a new line before the sentence, plus a note sign, like:

If you have devices with very similar *configuration dictionaries* you can store the shared blocks in one or more reusable templates which will be used as a base to build the final configuration.
 
.. note::
    when using multiple templates, their order is important.
    Templates that come afterwards overrides the templates that come before it.

[coveralls]

Coverage remained the same at 100.0% when pulling 7d5fcfd on ritwickdsouza:ordering-templates into 5fc47f9 on openwisp:master.

[ritwickdsouza]

@nemesisdesign Any more changes required ?

[nemesifier]

Looks good.