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/primitives/message-dispatch/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "bp-message-dispatch"
+description = "Primitives of bridge messages dispatch modules."
+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-runtime = { path = "../runtime", default-features = false }
+codec = { package = "parity-scale-codec", version = "2.0.0", default-features = false }
+
+[features]
+default = ["std"]
+std = [
+	"bp-runtime/std",
+	"codec/std",
+]