add simple explanation about test cases

[sssvip]

add simple explanation about test cases, very useful for golang fresher

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[coveralls]

Coverage remained the same at 95.76% when pulling 2302427 on sssvip:master into 651680c on cweill:master.

[cweill]

I agree that it would be a better UX to add some examples of how this works. I'll need to think about this a bit. We need to at least clean up the examples a bit.