Add unit testing example to README

[mkropat]

Unit testing is still relatively new to the shell scripting world. The average shell programmer isn't going to know what unit testing is for and when you would use it, so it's even more unlikely that they'd know why you would want to use spies and stubs.

There needs to be a concise yet complete introduction to shell unit testing with spies and fakes that not only explains how to use the functions in shpy, but it would also explain:

• why unit testing is useful
• how you go about unit testing shell scripts
• what problem spies and stubs solve
• how to use spies effectively

I feel like I know what needs to be said, but at this point I still need to think of a programming problem that:

• illustrates the relevant testing concepts
• is a problem that most programmers can relate to and understand
• is concise enough that we don't have to go into needless detail