docs: add a security model page for users

Readers asked whether the client can be trusted with their files. The
threat model in ARCHITECTURE.md is written for contributors, so
docs/SECURITY_MODEL.md summarises it for users: encryption comes from
the SDK and rPGP, sign-in uses SRP, what is stored where and how it is
protected, who can reach the daemon, every network connection, what the
client does not protect against, and how each package is built and
signed. It says that there has been no independent audit.

It is linked from the README, the documentation index and SECURITY.md.

docs/README.md

@@ -10,6 +10,7 @@
 | [Configuration](CONFIGURATION.md) | `config.json`, environment variables, file locations, the systemd service |
 | [Troubleshooting](TROUBLESHOOTING.md) | Diagnosing problems, common symptoms, starting over, reporting a bug |
 | [Recovering on a new computer](RECOVERY.md) | What survives a lost machine and how to restore your synced folders |
+| [Security model](SECURITY_MODEL.md) | What protects your files, what is kept on this computer, what the client connects to |
 | [Changelog](CHANGELOG.md) | What changed in each release |
 
 ## For contributors