security.txt: Add acknowledgments page to honour our security researches

The page will be linked in [https://torproject.org/.well-known/security.txt]

Details: ​https://tools.ietf.org/html/draft-foudil-securitytxt-04#section-3.4.1 Basically a place to honour the work of former / current security researchers.

This could also go into legacy/trac#25475 (moved).