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

SKIP_ME="### CT_NG SKIP ME ###"

skip_me=$(($(cat "${0}" |@@grep@@ --binary-files=text -o -b -E "^${SKIP_ME}$" |cut -d : -f 1)+${#SKIP_ME}+1))

dd if="${0}" bs=${skip_me} skip=1 2>/dev/null |bzcat

exit 0

### CT_NG SKIP ME ###