[docs] Add gcov instructions in readme and contributing

Author: KFilipek

CONTRIBUTING.md

@@ -202,3 +202,18 @@ New dependency: dependency_name
 license: SPDX license tag
 origin: https://dependency_origin.com
 ```
+## Code coverage
+
+After adding new functionality add tests and check coverage before and after change.
+To do that, enable coverage instrumentation by enabling USE_GCOV flag in CMake.
+Example flow may look like:
+
+```bash
+$ cmake -B build -DUSE_GCOV=1 -DCMAKE_BUILD_TYPE=Debug
+$ cmake --build build -j
+$ cd build
+$ ctest
+$ apt install lcov
+$ lcov --capture --directory . --output-file coverage.info
+$ genhtml -o html_report coverage.info
+```

README.md

@@ -123,6 +123,7 @@ List of options provided by CMake:
 | USE_TSAN | Enable ThreadSanitizer checks | ON/OFF | OFF |
 | USE_MSAN | Enable MemorySanitizer checks | ON/OFF | OFF |
 | USE_VALGRIND | Enable Valgrind instrumentation | ON/OFF | OFF |
+| USE_GCOV | Enable gcov support | ON/OFF | OFF |
 
 ## Architecture: memory pools and providers