contrib/hooks: add sample post-receive hook using agefile

One of the most frequent questions on the mailing list relates to the
idle time in the repository list.  The answer to this is to use the
"agefile" feature to calculate the time of the last change whenever the
repository receives changes.

Add a sample post-receive hook in a new "contrib" directory so that we
can just point people at the repository in the future.

Signed-off-by: John Keeping <john@keeping.me.uk>