Update docs to reflect community fork and recent changes

- Rewrite README: update all refs from shenxn to dancwilliams, remove deb
  build references, document what changed from upstream, simplify tags
  section, update all docker run and compose commands, add architecture
  table, add version update automation section, add credits
- Update SECURITY.md: replace shenxn contact with dancwilliams GitHub
  issue reporting, clarify scope (container vs bridge itself)
- Update docker-compose.yml: point to dancwilliams/protonmail-bridge,
  remove deprecated version key

docker-compose.yml

@@ -1,14 +1,13 @@
-version: '2.1'
-
 services:
   protonmail-bridge:
-    image: shenxn/protonmail-bridge
+    image: dancwilliams/protonmail-bridge:latest
     ports:
       - 1025:25/tcp
       - 1143:143/tcp
     restart: unless-stopped
     volumes:
       - protonmail:/root
+
 volumes:
   protonmail:
     name: protonmail