showstack: add an extension docstring

Now, running `hg help showstack` will give details on how to use the extension.

contrib/showstack.py

@@ -1,20 +1,22 b''
 # showstack.py - extension to dump a Python stack trace on signal
 #
 # binds to both SIGQUIT (Ctrl-\) and SIGINFO (Ctrl-T on BSDs)
+"""dump stack trace when receiving SIGQUIT (Ctrl-\) and SIGINFO (Ctrl-T on BSDs)
+"""
 
 from __future__ import absolute_import
 import signal
 import sys
 import traceback
 
 def sigshow(*args):
     sys.stderr.write("\n")
     traceback.print_stack(args[1], limit=10, file=sys.stderr)
     sys.stderr.write("----\n")
 
 def extsetup(ui):
     signal.signal(signal.SIGQUIT, sigshow)
     try:
         signal.signal(signal.SIGINFO, sigshow)
     except AttributeError:
         pass