Add bug reporting instructions to readme
French update
Update README.md

To avoid maintaining in several places I link the contributor guide here. I did extend and rework the guidelines for bugreports a bit.
fix some issues shown on Zenphoto site
Merge remote-tracking branch 'origin/master' into 1.4.6

Conflicts:
	zp-core/locale/fr_FR/LC_MESSAGES/zenphoto.mo
	zp-core/locale/fr_FR/LC_MESSAGES/zenphoto.po

	modified:   README.md
	modified:   zp-core/zp-extensions/cacheManager/cacheDBImages.php
	modified:   zp-core/zp-extensions/rating/update.php