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/.dependabot/config.yml

@@ -0,0 +1,20 @@
+version: 1
+update_configs:
+  - package_manager: "rust:cargo"
+    directory: "/"
+    update_schedule: "weekly"
+    ignored_updates:
+      - match:
+          dependency_name: "sp-*"
+      - match:
+          dependency_name: "sc-*"
+      - match:
+          dependency_name: "substrate-*"
+      - match:
+          dependency_name: "frame-*"
+      - match:
+          dependency_name: "pallet-*"
+    automerged_updates:
+      - match:
+          update_type: "all"
+    version_requirement_updates: "auto"