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/scripts/run-openethereum-node.sh

@@ -0,0 +1,11 @@
+#!/bin/bash
+
+# This script assumes that an OpenEthereum build is available. The repo
+# should be at the same level as the `parity-bridges-common` repo.
+
+RUST_LOG=rpc=trace,txqueue=trace,bridge-builtin=trace \
+../openethereum/target/debug/openethereum \
+    --config="$(pwd)"/deployments/dev/poa-config/poa-node-config \
+    --node-key=arthur \
+    --engine-signer=0x005e714f896a8b7cede9d38688c1a81de72a58e4 \
+    --base-path=/tmp/oe-dev-node \