docs/overview.txt: Provide short mercurial HOWTO

The usage of hg mq is imho not very well documented.
Give a short intro for the most important use cases
for contributions to ct-ng.

yann.morin.1998@anciens.enib.fr:
Slightly rewrote the explanations for the introductory message.

 #!/bin/sh

 # Needed for systems without gettext

 $* -xc -o /dev/null - > /dev/null 2>&1 << EOF

 #include <libintl.h>

 int main()

 {

 	gettext("");

 	return 0;

 }

 EOF

 if [ ! "$?" -eq "0"  ]; then

 	echo -DKBUILD_NO_NLS;

 fi