Add notes on debugging with gdb.

CONTRIBUTING.md

@@ -145,6 +145,30 @@ Just do the same procedure to create a new, merged pill before
 committing the merge. Otherwise, just make sure to use the correct
 `urbit.pill` for each commit.
 
+## Debug urbit with `gdb`
+
+Follow the build instructions in README.md but run `make` with argument `DEBUG=yes`:
+
+(If you've already built urbit first run `make clean`.)
+
+    make DEBUG=yes
+
+Run `gdb`, while loading `bin/urbit` and its symbol table:
+
+    gdb bin/urbit
+
+Set a breakpoint on `main()` (optional):
+
+    break main
+
+Run your urbit comet `mycomet`:
+
+    run mycomet
+
+Continue from the breakpoint on `main()`:
+
+    continue
+
 ## What to work on
 
 If you are not thinking of contributing with a specific goal in mind,

Makefile

@@ -70,10 +70,18 @@ endif
 INCLUDE=include
 MDEFINES=-DU3_OS_$(OS) -DU3_OS_ENDIAN_$(ENDIAN) -D U3_LIB=\"$(LIB)\"
 
+DEBUG=no
+
+ifeq ($(DEBUG),yes)
+DEBUGFLAGS=-g
+else
+DEBUGFLAGS=-O3
+endif
+
 # NOTFORCHECKIN - restore -O3
 # 	-DGHETTO \
 #   -DHUSH
-CFLAGS= $(COSFLAGS) -O3 -ffast-math \
+CFLAGS= $(COSFLAGS) $(DEBUGFLAGS) -ffast-math \
 	-funsigned-char \
 	-I/usr/local/include \
 	-I/opt/local/include \