Add link to Google Test Primer docs in README.md

On google code, the main documentation pages were the "Google Test Primer" content. Currently it's hard to find that content on github, so add a direct link in the main README.md.

Author: pjaytycy

README.md

@@ -14,6 +14,9 @@ mailing list for questions, discussions, and development.  There is
 also an IRC channel on OFTC (irc.oftc.net) #gtest available.  Please
 join us!
 
+Getting started information for **Google Test** is available in the 
+[Google Test Primer](googletest/docs/Primer.md) documentation.
+
 **Google Mock** is an extension to Google Test for writing and using C++ mock
 classes.  See the separate [Google Mock documentation](googlemock/README.md).