Update README.md

README.md

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-# classic-gazebo-gtest
-GTest vendored inside Classic Gazebo moved to a separate repo to simplify usage with FetchContent.
+# gazebo-classic-gtest
+
+[Gazebo Classic](https://github.com/osrf/gazebo) provides the `gazebo::ServerFixture` class in the `gazebo/test/ServerFixture.hh` header, to simplify developing tests on top of its libraries.
+
+Unfortunatly, to use it you **must** use exactly the same version of googletest that Gazebo vendors in its [`gazebo/test/gtest`](https://github.com/osrf/gazebo/tree/gazebo11/test/gtest) directory. This repo contains exactly a copy of that directory, in a standalone git repo to simplify its inclusion in other repos via [CMake's FetchContent module](https://cmake.org/cmake/help/latest/module/FetchContent.html).