Update documentation

Add "Debugging Facilities" section to README.md

Author: kickasson

.gitignore

@@ -4,3 +4,4 @@ qtest
 *.dSYM
 .vscode
 .devcontainer
+core*

README.md

@@ -83,6 +83,7 @@ Tools for evaluating your queue code
 * Makefile : Builds the evaluation program `qtest`
 * README.md : This file
 * scripts/driver.py : The driver program, runs `qtest` on a standard set of traces
+* scripts/debug.py : The helper program for GDB, executes qtest without SIGALRM and/or analyzes generated core dump file.
 
 Helper files
 * console.{c,h} : Implements command-line interpreter for qtest
@@ -97,6 +98,49 @@ Trace files
   * XX is the trace number (1-15).  CAT describes the general nature of the test.
 * traces/trace-eg.cmd : A simple, documented trace file to demonstrate the operation of `qtest`
 
+## Debugging Facilities
+
+Before using GDB debug `qtest`, there are some routine instructions need to do. The script `scripts/debug.py` covers these instructions and provides basic debug function. 
+```
+$ scripts/debug.py -h
+usage: debug.py [-h] [-d | -a]
+
+optional arguments:
+  -h, --help     show this help message and exit
+  -d, --debug    Enter gdb shell
+  -a, --analyze  Analyze the core dump file
+```
+* Enter GDB without interruption by **SIGALRM**.
+```
+$ scripts/debug.py -d
+Reading symbols from lab0-c/qtest...done.
+Signal        Stop	Print	Pass to program	Description
+SIGALRM       No	No	No		Alarm clock
+Starting program: lab0-c/qtest 
+cmd> 
+```
+* When `qtest` encountered **Segmentation fault** while it ran outside GDB, we could invoke GDB in the post-mortem debugging mode to figure out the bug.
+
+  The core dump file was created in the working directory of the `qtest`.
+  * Allow the core dumps by using shell built-in command **ulimit** to set core file size.
+  ```
+  $ ulimit -c unlimited
+  $ ulimit -c
+  unlimited
+  ```
+  * Enter GDB and analyze
+  ```
+  $ scripts/debug.py -a
+  Reading symbols from lab0-c/qtest...done.
+  [New LWP 9424]
+  Core was generated by `lab0-c/qtest'.
+  Program terminated with signal SIGSEGV, Segmentation fault.
+  #0 ...
+  #1 ... (backtrace information)
+  #2 ...
+  (gdb) 
+  ```
+
 ## License
 
 `lab0-c`is released under the BSD 2 clause license. Use of this source code is governed by