doc: hint about GIT_DEBUGGER in CodingGuidelines

We check for a handy environment variable GIT_DEBUGGER when running via
bin-wrappers/, but this feature is undocumented. Add a hint to how to
use it into the CodingGuidelines (which is where other useful
environment settings like DEVELOPER are documented).

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/CodingGuidelines

@@ -412,6 +412,12 @@ For C programs:
    must be declared with "extern" in header files. However, function
    declarations should not use "extern", as that is already the default.
 
+ - You can launch gdb around your program using the shorthand GIT_DEBUGGER.
+   Run `GIT_DEBUGGER=1 ./bin-wrappers/git foo` to simply use gdb as is, or
+   run `GIT_DEBUGGER="<debugger> <debugger-args>" ./bin-wrappers/git foo` to
+   use your own debugger and arguments. Example: `GIT_DEBUGGER="ddd --gdb"
+   ./bin-wrappers/git log` (See `wrap-for-bin.sh`.)
+
 For Perl programs:
 
  - Most of the C guidelines above apply.