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/add_license.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+
+PAT_GPL="^// Copyright.*If not, see <http://www.gnu.org/licenses/>\.$"
+PAT_OTHER="^// Copyright"
+
+SCRIPTS_DIR=$( cd "$(dirname "${BASH_SOURCE[0]}")" ; pwd -P )
+
+for f in $(find . -type f | egrep '\.(c|cpp|rs)$'); do
+	HEADER=$(head -16 $f)
+	if [[ $HEADER =~ $PAT_GPL ]]; then
+		BODY=$(tail -n +17 $f)
+		cat $SCRIPTS_DIR/license_header > temp
+		echo "$BODY" >> temp
+		mv temp $f
+	elif [[ $HEADER =~ $PAT_OTHER ]]; then
+		echo "Other license was found do nothing"
+	else
+		echo "$f was missing header"
+		cat $SCRIPTS_DIR/license_header $f > temp
+		mv temp $f
+	fi
+done