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/local-scripts/bridge-entrypoint.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+set -xeu
+
+# This will allow us to run whichever binary the user wanted
+# with arguments passed through `docker run`
+# e.g `docker run -it rialto-bridge-node-dev --dev --tmp`
+/home/user/$PROJECT $@