Add SimpleX as Signal alternative (#130)

After confirmation in #129, and after the release of the SimpleX mobile clients,
adding SimpleX as a real Signal alternative seems like a good idea. There are
mostly good things to say, in my opinion (I'm not affiliated), but I've still tried to
remain neutral.

why_not_signal.md

@@ -32,6 +32,8 @@
     + [Briar](#briar)
       - [Caveats](#caveats-1)
     + [Jitsi](#jitsi)
+    + [SimpleX](#simplex)
+      - [Caveats](#simplex-caveats)
   * [Conclusions](#conclusions)
 
 <!-- tocstop -->
@@ -245,6 +247,15 @@ Briar is newer, and lacks too many features to be usable for large organizations
 
 [Jitsi](https://jitsi.org/) is a self-hostable, open source, video-conferencing solution, that can work as an alternative to Zoom. It has a web app, and native apps for desktop, iOS, and android. Matrix currently bundles in Jitsi for use for its video group calls.
 
+### SimpleX
+
+[SimpleX](https://github.com/simplex-chat/simplex-chat#readme) is a privacy-first messaging protocol and platform. It is open source, anonymous (no identifiers assigned to users), and end-to-end encrypted. The server is fully self-hostable, and the CLI client, iOS client, and Android client can change from the default set of servers run by the SimpleX project in a menu. The Android app is available on F-Droid.
+
+#### SimpleX Caveats
+
+- The iOS and Android clients don't use the app store-run push notification system for privacy reasons; receiving messages can be delayed when the app isn't open.
+- There isn't currently a web-based client because the SimpleX project assesses that web has too many attack vectors.
+
 ## Conclusions
 
 I've outlined here why it's imperative that activist communities migrate away from Signal. We shouldn't be daunted by the inertia that seems to plague general adoption of new chat applications; if our organizations are disciplined enough, and care enough about secure communications, moving to them should be made a priority.