Add BUILD_TESTS Flag to Readme for CMake [skip ci] (#5973)

See title as i was developing on Linux didn't found the option for
building the test binary. With some feedback and examination of the
Actions i found that the BUILD_TESTS flag should be enabled. Added that
to Readme.

readme.md

@@ -64,6 +64,9 @@ For rules support (requires pcre) use the flag.
 For release builds it is recommended that you use:
 -DUSE_MATCHCOMPILER=ON
 
+For building the tests use the flag.
+-DBUILD_TESTS=ON
+
 Using cmake you can generate project files for Visual Studio,XCode,etc.
 
 #### Building a specific configuration