D9577: Document ".okular" filename extension

[Yuri Chornoivan]

yurchor accepted this revision.
This revision is now accepted and ready to land.

REPOSITORY
  R223 Okular

BRANCH
  master

REVISION DETAIL
  https://phabricator.kde.org/D9577

To: rkflx, lueck, yurchor
Cc: #okular, michaelweghorn, gassaf, ngraham, aacid