Link to the thread safety documentation from docs/Readme.md

Author: horenmar

docs/Readme.md

@@ -32,6 +32,7 @@ Once you're up and running consider the following reference material.
 * [CMake integration](cmake-integration.md#top)
 * [Tooling integration (CI, test runners, other)](ci-and-misc.md#top)
 * [Known limitations](limitations.md#top)
+* [Thread safety in Catch2](thread-safety.md#top)
 
 **Other:**
 * [Why Catch2?](why-catch.md#top)