Better README

[lasseebert]

The README should describe different scenarios where Kase can help rewrite the code to be either more safe or more readable. Or both.

Examples:

• Matching a Rack response by status
• Avoid assignment in if condition, but match with Kase instead
• Replacing case my_var ... else raise "unexpected my_var" end with Kase that matches just a single variable
• The obvious, already documented [status, result] like return value matching

More good examples?

Also add section about where the native case is better and Kase should not be used.