Add some advice about using gdb to CONTRIBUTING.

CONTRIBUTING.rst

@@ -32,6 +32,19 @@ For development/debugging cycles, the ``configure`` option
 off optimizations and function inlining, so that a debugger will step
 through the code as expected.
 
+Experience has shown that adding ``-ggdb3`` to ``CFLAGS`` is
+beneficial if you need to examine the VMOD with the gdb debugger. The
+shared object for a VMOD is loaded from a directory relative to the
+Varnish home directory (by default ``/usr/local/var/$INSTANCE`` for
+development builds). A debugger needs to locate the shared object from
+that relative path to load its symbols, so the Varnish home directory
+should be the current working directory when the debugger is run. For
+example::
+
+  # To run gdb and examine a coredump
+  $ cd /usr/local/var/myinstance
+  $ gdb /usr/local/sbin/varnishd /path/to/coredump
+
 By default, the VMOD is built with the stack protector enabled
 (compile option ``-fstack-protector``), but it can be disabled with
 the ``configure`` option ``--disable-stack-protector``.