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/relays/utils/Cargo.toml

@@ -0,0 +1,22 @@
+[package]
+name = "relay-utils"
+version = "0.1.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+edition = "2018"
+license = "GPL-3.0-or-later WITH Classpath-exception-2.0"
+
+[dependencies]
+ansi_term = "0.12"
+async-std = "1.6.5"
+async-trait = "0.1.40"
+backoff = "0.2"
+env_logger = "0.8.2"
+futures = "0.3.5"
+log = "0.4.11"
+num-traits = "0.2"
+sysinfo = "0.15"
+time = "0.2"
+
+# Substrate dependencies
+
+substrate-prometheus-endpoint = { git = "https://github.com/paritytech/substrate.git", branch = "master" }