Add day 13 debugging exercises and README

Add src/main.py with the six debugging exercises (describe the problem,
reproduce the bug, play computer, fix the errors, print is your friend,
use a debugger), each with its fix noted. Document them in the README.

README.md

@@ -1,2 +1,35 @@
 # 013-debugging
 
+Day 13 of Udemy's *100 Days of Code: Python Pro Bootcamp*: finding and fixing
+bugs in Python code.
+
+All exercises live in [`src/main.py`](src/main.py), each commented out with the
+fix noted beside it. Uncomment one at a time to run it.
+
+## Exercises
+
+| Exercise | Bug | Fix |
+| --- | --- | --- |
+| Describe the Problem | `range(1, 20)` never reaches 20, because the upper bound is excluded | Use `range(1, 21)` |
+| Reproduce the Bug | `dice_imgs[dice_num]` raises `IndexError` when the roll is 6 | Index with `dice_num - 1` |
+| Play Computer | A birth year of exactly 1994 matches no branch | Use `>=` in the `elif` |
+| Fix the Errors | `input()` returns a string, so `age > 18` fails; the `print` is not indented | Convert with `int()`; indent the `print` |
+| Print is Your Friend | `==` compares instead of assigning, so `word_per_page` stays 0 | Use `=` |
+| Use a Debugger | `b_list.append(new_item)` sits outside the loop, so only the last item is kept | Indent it into the loop |
+
+## Debugging approach
+
+1. Describe the problem in your own words.
+2. Reproduce the bug.
+3. Play computer: step through the code by hand.
+4. Fix the errors the interpreter reports.
+5. Use `print()` to inspect values.
+6. Use a debugger to step through execution.
+
+## Running
+
+Requires Python 3.
+
+```bash
+python src/main.py
+```