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 $@

polkadot/bridges/deployments/local-scripts/relay-millau-to-rialto.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+
+# A script for relaying Millau headers to the Rialto chain.
+#
+# Will not work unless both the Rialto and Millau are running (see `run-rialto-node.sh`
+# and `run-millau-node.sh).
+
+RUST_LOG=bridge=debug \
+./target/debug/substrate-relay init-bridge millau-to-rialto \
+	--millau-host localhost \
+	--millau-port 9945 \
+	--rialto-host localhost \
+	--rialto-port 9944 \
+	--rialto-signer //Alice \
+
+sleep 5
+RUST_LOG=bridge=debug \
+./target/debug/substrate-relay relay-headers millau-to-rialto \
+	--millau-host localhost \
+	--millau-port 9945 \
+	--rialto-host localhost \
+	--rialto-port 9944 \
+	--rialto-signer //Alice \
+	--prometheus-host=0.0.0.0

polkadot/bridges/deployments/local-scripts/relay-rialto-to-millau.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+
+# A script for relaying Rialto headers to the Millau chain.
+#
+# Will not work unless both the Rialto and Millau are running (see `run-rialto-node.sh`
+# and `run-millau-node.sh).
+
+RUST_LOG=bridge=debug \
+./target/debug/substrate-relay init-bridge rialto-to-millau \
+	--millau-host localhost \
+	--millau-port 9945 \
+	--rialto-host localhost \
+	--rialto-port 9944 \
+	--millau-signer //Alice \
+
+sleep 5
+RUST_LOG=bridge=debug \
+./target/debug/substrate-relay relay-headers rialto-to-millau \
+	--millau-host localhost \
+	--millau-port 9945 \
+	--rialto-host localhost \
+	--rialto-port 9944 \
+	--millau-signer //Alice \
+	--prometheus-host=0.0.0.0

polkadot/bridges/deployments/local-scripts/run-millau-node.sh

@@ -0,0 +1,8 @@
+#!/bin/bash
+
+# Run a development instance of the Millau Substrate bridge node.
+
+RUST_LOG=runtime=trace \
+./target/debug/millau-bridge-node --dev --tmp \
+    --rpc-cors=all --unsafe-rpc-external --unsafe-ws-external \
+    --port 33044 --rpc-port 9934 --ws-port 9945 \

polkadot/bridges/deployments/local-scripts/run-rialto-node.sh

@@ -0,0 +1,8 @@
+#!/bin/bash
+
+# Run a development instance of the Rialto Substrate bridge node.
+
+RUST_LOG=runtime=trace \
+    ./target/debug/rialto-bridge-node --dev --tmp \
+    --rpc-cors=all --unsafe-rpc-external --unsafe-ws-external \
+    --port 33033 --rpc-port 9933 --ws-port 9944 \