README_DEVELOPERS: Debugging a tool needs "--tool=..."

Otherwise the function redirections / client requests seem
to get confused. Symptoms are error messages such as
--3031-- VG_USERREQ__CLIENT_CALL1: func=0x0

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@12852 a5019735-40e9-0310-863c-91ae7b9d1cf9
1 file changed