add gdb attach process to test README

Author: instagibbs

test/README.md

@@ -155,6 +155,26 @@ import pdb; pdb.set_trace()
 anywhere in the test. You will then be able to inspect variables, as well as
 call methods that interact with the bitcoind nodes-under-test.
 
+If further introspection of the bitcoind instances themselves becomes
+necessary, this can be accomplished by first setting a pdb breakpoint
+at an appropriate location, running the test to that point, then using
+`gdb` to attach to the process and debug.
+
+For instance, to attach to `self.node[1]` during a run:
+
+```bash
+2017-06-27 14:13:56.686000 TestFramework (INFO): Initializing test directory /tmp/user/1000/testo9vsdjo3
+```
+
+use the directory path to get the pid from the pid file:
+
+```bash
+cat /tmp/user/1000/testo9vsdjo3/node1/regtest/bitcoind.pid
+gdb /home/example/bitcoind <pid>
+```
+
+Note: gdb attach step may require `sudo`
+
 ### Util tests
 
 Util tests can be run locally by running `test/util/bitcoin-util-test.py`.