Add a clarification concerning to bounties and sponsorships

[sjehuda]

Good day!

I advise to add a section that indicates of the possibility for adding support for OMEMO, provided that the developers be granted funds to do so, or that someone else would kindly be responsible to add that feature. See BombusMod/BombusMod#125 (comment)

Writing that a project has categorically refused to add OMEMO, is utterly misleading.