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/send-message.sh

@@ -0,0 +1,35 @@
+#!/bin/bash
+
+# Used for manually sending a message to a running network.
+#
+# You could for example spin up a full network using the Docker Compose files
+# we have (to make sure the message relays are running), but remove the message
+# generator service. From there you may submit messages manually using this script.
+
+case "$1" in
+	remark)
+		RUST_LOG=runtime=trace,substrate-relay=trace,bridge=trace \
+		./target/debug/substrate-relay send-message millau-to-rialto \
+			--millau-host localhost \
+			--millau-port 20044 \
+			--millau-signer //Dave \
+			--rialto-signer //Dave \
+			--lane 00000000 \
+			--origin Target \
+			remark \
+		;;
+	transfer)
+		RUST_LOG=runtime=trace,substrate-relay=trace,bridge=trace \
+		./target/debug/substrate-relay send-message millau-to-rialto \
+			--millau-host localhost \
+			--millau-port 20044 \
+			--millau-signer //Dave \
+			--rialto-signer //Dave \
+			--lane 00000000 \
+			--origin Target \
+			transfer \
+			--amount 100000000000000 \
+			--recipient 5DZvVvd1udr61vL7Xks17TFQ4fi9NiagYLaBobnbPCP14ewA \
+		;;
+	*) echo "A message type is require. Supported messages: remark, transfer."; exit 1;;
+esac