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/ethereum-client/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name = "relay-ethereum-client"
+version = "0.1.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+edition = "2018"
+license = "GPL-3.0-or-later WITH Classpath-exception-2.0"
+
+[dependencies]
+bp-eth-poa = { path = "../../primitives/ethereum-poa" }
+codec = { package = "parity-scale-codec", version = "2.0.0" }
+headers-relay = { path = "../headers-relay" }
+hex-literal = "0.3"
+jsonrpsee = { git = "https://github.com/svyatonik/jsonrpsee.git", branch = "shared-client-in-rpc-api", default-features = false, features = ["http"] }
+libsecp256k1 = { version = "0.3.4", default-features = false, features = ["hmac"] }
+log = "0.4.11"
+relay-utils = { path = "../utils" }
+web3 = { version = "0.15", git = "https://github.com/tomusdrw/rust-web3.git", branch ="td-ethabi", default-features = false }