update README: add link to bug tracker
doc: update README, COPYING
 * get list of committers with git shortlog -sne
 * Add copyright from http://sebsauvage.net/wiki/doku.php?id=php:shaarli, http://blog.idleman.fr/une-peau-neuve-pour-shaarli-de-sebsauvage/
 * reword copyright
doc: README:
 * add install/upgrade instructions
 * move related software to wiki