Describe the different grants in README.txt

Now that the module is providing two different types of grants, it makes sense to bulk up the README.txt to explain that to the user.

Also, we should mention something about the difference between what happens when you go from 6.x-2.6 to anything greater than 6.x-2.6 because:

* Before: node author has no grants to her nodes
* After: node author has read, update, delete to her nodes

So sites that were depending on the author not being able to read, update, delete their nodes will need to change the default configuration.