Improvement of the Installation and the Example

Though, I could figure it out - based on the source code and the test file - it would be nice, if the Readme.md would be clearer about the installation process and the example.

For the installation add the hint:

`
INSTALLED_APPS = (
...
'xorformfields',
)
`

And in the example please add the relevant python import lines so that a copy&paste of the example leads to working code.

Thanks.