Add prerequisite for test adapter nuget package (#102)

#33 (comment)

Author: tonyhallett

README.md

@@ -5,6 +5,11 @@
 Download this extension from the [Visual Studio Market Place](https://marketplace.visualstudio.com/items?itemName=FortuneNgwenya.FineCodeCoverage)
 or get the [CI build](https://www.vsixgallery.com/extension/fcc-f339fe606-9d51-4fca-895c-d50375137b62).  Previous versions can be obtained from [here](https://ci.appveyor.com/project/FortuneN/finecodecoverage).
 
+---------------------------------------
+Prerequisites
+
+Only that the test adapters are nuget packages.  For instance, the NUnit Test Adapter extension is not sufficient.
+
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 ### <a href="https://www.youtube.com/watch?v=Rae5bTE2D3o" target="_blank">Watch Introduction Video</a>