build CHANGE be mroe strict for compiler to build code coverage and add some docs in README

Code coverage process as used works well with gcc, but may cause
problems with other compilers. If the compiler is not gcc, do not enable
the coverage target by default even in debug build type and in case it
is explicitly enabled, print warning to let user know.
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index e7b4e8b..d79996d 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -29,7 +29,7 @@
     endif()
 
     if(NOT CMAKE_COMPILER_IS_GNUCC)
-        message(WARNING "Compiler is not gcc/g++! Building code coverage report may not work!")
+        message(WARNING "Compiler is not gcc! Coverage may break the tests!")
     endif()
 
     if(ENABLE_COVERAGE)