doc: Added running functional tests in valgrind

Author: elichai

doc/developer-notes.md

@@ -250,6 +250,7 @@ $ valgrind --suppressions=contrib/valgrind.supp src/test/test_bitcoin
 $ valgrind --suppressions=contrib/valgrind.supp --leak-check=full \
       --show-leak-kinds=all src/test/test_bitcoin --log_level=test_suite
 $ valgrind -v --leak-check=full src/bitcoind -printtoconsole
+$ ./test/functional/test_runner.py --valgrind
 ```
 
 ### Compiling for test coverage