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/substrate-client/src/lib.rs

@@ -0,0 +1,55 @@
+// Copyright 2019-2020 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity Bridges Common is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Tools to interact with (Open) Ethereum node using RPC methods.
+
+#![warn(missing_docs)]
+
+mod chain;
+mod client;
+mod error;
+mod rpc;
+mod sync_header;
+
+pub mod guard;
+pub mod headers_source;
+
+pub use crate::chain::{BlockWithJustification, Chain, ChainWithBalances, TransactionSignScheme};
+pub use crate::client::{Client, JustificationsSubscription, OpaqueGrandpaAuthoritiesSet};
+pub use crate::error::{Error, Result};
+pub use crate::sync_header::SyncHeader;
+pub use bp_runtime::{BlockNumberOf, Chain as ChainBase, HashOf, HeaderOf};
+
+/// Header id used by the chain.
+pub type HeaderIdOf<C> = relay_utils::HeaderId<HashOf<C>, BlockNumberOf<C>>;
+
+/// Substrate-over-websocket connection params.
+#[derive(Debug, Clone)]
+pub struct ConnectionParams {
+	/// Websocket server hostname.
+	pub host: String,
+	/// Websocket server TCP port.
+	pub port: u16,
+}
+
+impl Default for ConnectionParams {
+	fn default() -> Self {
+		ConnectionParams {
+			host: "localhost".into(),
+			port: 9944,
+		}
+	}
+}