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/dev/poa-config/poa-node-config

@@ -0,0 +1,17 @@
+[parity]
+chain = "./deployments/dev/poa-config/poa.json"
+keys_path = "./deployments/dev/poa-config/keys"
+no_persistent_txqueue = true
+
+[account]
+password = ["./deployments/dev/poa-config/pass"]
+
+[rpc]
+apis = ["all"]
+cors = ["moz-extension://*", "chrome-extension://*"]
+
+[mining]
+force_sealing = true
+
+[misc]
+unsafe_expose = true