Add a reference to Mooncake's user-friendly warning/error message to README/docs

[yebai]

Mooncake produces user-friendly messages that help users to

• identify Mooncake-related issues/bugs quicker (e.g. lack of rules, incorrect use of functionality)
• give straightforward tips on what code cannot be differentiated, and help user revise their (primal) code if it has issues
• others...

This is very useful for reducing the development and maintenance burden of a (language-level, general-purpose) autograd package, so it should be more visible in README or docs.