Improve introduction docs

docs/index.rst

@@ -4,15 +4,22 @@ django-payments
 This library is a Django app for handling payments with a few different payment
 providers.
 
-It includes support for a few different payment providers using a unified API.
+When using ``django-payments``, you integrate with the Python API the library
+exposes, and the library handles all the communication with the payment
+processor. The most obvious use case is implementing online payments on
+e-commerce websites, though other unusual cases exist.
+
+Because it provides a common API for different providers, it once one provider
+is working, it's trivial to add support for others onto a same codebase.
 
 General design
 --------------
 
 A single abstract model is included: :class:`payments.models.BasePayment`.
 Applications using this library need to subclass it and implement a few specific
-method. It is also possible to domain-specific fields that the may be required
-(e.g.: foreign keys to a purchase, or user).
+method (see the class docs for details). Subclasses can also include their own
+additional fields that may be required for payments (e.g.: a foreign key to a
+purchase, or a user). This is up to the developer.
 
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