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/update_substrate.sh

@@ -0,0 +1,10 @@
+#!/bin/sh
+
+# One-liner to update between Substrate releases
+# Usage: ./update_substrate.sh 2.0.0-rc6 2.0.0
+set -xeu
+
+OLD_VERSION=$1
+NEW_VERSION=$2
+
+find . -type f -name 'Cargo.toml' -exec sed -i '' -e "s/$OLD_VERSION/$NEW_VERSION/g" {} \;