Secuchart addition

Created by: pterocles

Needs to be said that Secuchart https://bkil.gitlab.io/secuchart/ is a fantastic resource when deciding between chat/IM clients. It involves a lot of things such as the safety of the protocol (audits), E2EE status, PFS status, and whether or not there are F-droid APKs and where to see the source, etc. I've recommended this resource to many already with great feedback. I would like to place it in the list of additional resources before the comparison in the guide, so people can see a updated and maintained chart for help choosing an app to communicate safely with.

All credit to bkil on Matrix.