extension.json
* Add myself (Bartosz Dziewoński) and Matthias Mullie as principal authors.
* Add Ian Baker as principal author (was credited in README only).

README
* Remove outdated list of authors.

en.json
* Remove outdated list of authors.

.mailmap
* Map of canonical names and emails for `git log` and other scripts.
  Adapted from MediaWiki core.

CREDITS
* Credit everyone who has ever contributed to the extension. Generated
  using maintenance/updateCredits.php in MediaWiki core.

Change-Id: I181dda3ca3ef3f43e863482057b4949c1bc32e9b