add functions insturmentation example

Chapter02/README.md

@@ -3,8 +3,12 @@
 This folder contains examples in C++ from second chapter. 
 
 The folder contains following examples:
+- instrument_functions.cpp
 - vector_dyn_mem.cpp
 - vector_dyn_mem_reserve.cpp
 - vector_pmr.cpp
 - terminate_handler.cpp
 - new_deleted.cpp
+
+## Functions instrumentation in GCC
+Compile instrument_functions.cpp example using compiler flag -finstrument-functions