Usage documentation

[moodmosaic]

We should document the basic usage scenarios of rv. This should include:

• Difference between a stateless test (property) and a stateful test (invariant).
• Example of a property test.
• Example of an invariant test.
• Explanation of what the context is used for.
• Mention that rv tests properties probabilistically.
• (Once/If merged) Documentation of Custom manifest file support #107 about custom manifest support.
• (Once/If merged) Documentation of Add printed events to the context for assertions #59 on post-assertions defined by users.
• Explanation of the 4GB memory limit on Wasm programs, as discussed here.
• Why you should not set a higher number of runs than 50K, as discussed here.
• Future support for formal verification, which would check all paths exhaustively.

This will make it easier for users to understand rv, use it correctly, and avoid common mistakes.