Add generating *.gov files locally steps

docs/contributing.md

@@ -274,3 +274,10 @@ To auto-convert to the above syntax you can put this in an .emacs file:
     ;;
     (c-set-offset 'template-args-cont 'c-lineup-template-args)
 
+### Generating *.gcov files with LLVM/clang toolchain to check tests coverage
+
+* Ensure `llvm-cov` is available 
+* Build and link mapnik with `--coverage` and ensure optimisation is off and profile flags is present. For example pass following options to Scons: `CUSTOM_CXXFLAGS='--coverage -g -O0' LDFLAGS='--coverage'`
+* Run test(s) e.g `./test/unit/run`
+* To generate *.gcov file(s) run `llvm-cov gcov <path-to-cpp-file>` 
+* *.gcov files can be viewed in text editor