Explain services and modules

[jj5]

Tidy up the following notes and put them in the README.md.

A crypto service implements IKickassCrypto. There are four types of crypto service provided by this library, and implementers can define their own.

The crypto framework is a base class KickassCrypto which implements the IKickassCrypto interface; some parts of KickassCrypto are abstract and must be provided by implementations.

A use case suggests approaches to the number and types of secret keys that are used, implementers can add use cases; the two default use cases are:

• round-trip
• at-rest

A service locator finds a service for a particular use case, the library includes two service locators, one for each use case; the two default service locators are:

• kickass_round_trip()
• kickass_at_rest()

A module provides a crypto service for each use case, so by default that's two services per module, one for each use case.

This library provides two modules, Sodium and OpenSSL; as each module provides two services the full list of complete services supported by this library is:

• KickassSodiumRoundTrip
• KickassSodiumAtRest
• KickassOpenSslRoundTrip
• KickassOpenSslAtRest

Services for use cases within modules are designed to be independent because they use different secret keys; however if the services within a module did share secret keys then they should be mutually compatible. That is, if you configured the round-trip and at-rest secret keys to be the same, then any service in a module could be used to encrypt or decrypt any given plaintext or ciphertext. So the only thing that keeps services within a module independent should be the secret keys, otherwise services within a module should be mutually compatible.