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/bin/rialto/runtime/src/benches.rs

@@ -0,0 +1,37 @@
+// Copyright 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/>.
+
+//! We want to use a different validator configuration for benchmarking than what's used in Kovan
+//! or in our Rialto test network. However, we can't configure a new validator set on the fly which
+//! means we need to wire the runtime together like this
+
+use pallet_bridge_eth_poa::{ValidatorsConfiguration, ValidatorsSource};
+use sp_std::vec;
+
+pub use crate::kovan::{
+	genesis_header, genesis_validators, BridgeAuraConfiguration, FinalityVotesCachingInterval, PruningStrategy,
+};
+
+frame_support::parameter_types! {
+	pub BridgeValidatorsConfiguration: pallet_bridge_eth_poa::ValidatorsConfiguration = bench_validator_config();
+}
+
+fn bench_validator_config() -> ValidatorsConfiguration {
+	ValidatorsConfiguration::Multi(vec![
+		(0, ValidatorsSource::List(vec![[1; 20].into()])),
+		(1, ValidatorsSource::Contract([3; 20].into(), vec![[1; 20].into()])),
+	])
+}