Adding Bridges code as git subtree. (#2515)

* Add instructions.

* Squashed 'bridges/' content from commit 345e84a21

git-subtree-dir: bridges
git-subtree-split: 345e84a2146b56628e9888c9f5e129cb40e868a9

* Remove bridges workspace file to avoid confusing Cargo.

* Add some bridges primitives to Polkadot workspace.

* Improve docs.

polkadot/bridges/deployments/reverse-proxy/docker-compose.yml

@@ -0,0 +1,42 @@
+version: '2'
+services:
+  nginx-proxy:
+    image: jwilder/nginx-proxy
+    container_name: nginx-proxy
+    networks:
+      - nginx-proxy
+      - deployments_default
+    ports:
+      - "80:80"
+      - "443:443"
+    volumes:
+      - conf:/etc/nginx/conf.d
+      - vhost:/etc/nginx/vhost.d
+      - html:/usr/share/nginx/html
+      - dhparam:/etc/nginx/dhparam
+      - certs:/etc/nginx/certs:ro
+      - /var/run/docker.sock:/tmp/docker.sock:ro
+
+  letsencrypt:
+    image: jrcs/letsencrypt-nginx-proxy-companion
+    container_name: nginx-proxy-le
+    networks:
+      - nginx-proxy
+    volumes_from:
+      - nginx-proxy
+    volumes:
+      - certs:/etc/nginx/certs:rw
+      - /var/run/docker.sock:/var/run/docker.sock:ro
+
+volumes:
+  conf:
+  vhost:
+  html:
+  dhparam:
+  certs:
+
+networks:
+  nginx-proxy:
+    driver: bridge
+  deployments_default:
+    external: true