add default robots.txt for all wikis
Indexing /index.php doesn't make much sense - it provides page history
etc. which isn't really needed in search engines. Besides keeping search
engines "clean", it also avoids that search engines like YandexImages go
wild (260589 requests by YandexImages to index.php vs. 6300 requests
with other user agents on tr.o.o today!)
Our "real content" is available with speaking URLs (without index.php)
in the URL.