[readme] Link to explain what TAP is

I spent a few minutes trying to understand what "tap producing" meant. Maybe I'm not the only one?

Also, I capitalized it to help people like me infer it's an acronym.

readme.markdown

@@ -1,6 +1,6 @@
 # tape <sup>[![Version Badge][npm-version-svg]][package-url]</sup>
 
-tap-producing test harness for node and browsers
+[TAP](https://testanything.org/)-producing test harness for node and browsers
 
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