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/Cargo.lock

@@ -597,6 +597,58 @@ dependencies = [
  "once_cell",
 ]
 
+[[package]]
+name = "bp-kusama"
+version = "0.1.0"
+dependencies = [
+ "bp-message-lane",
+ "bp-runtime",
+ "frame-support",
+ "frame-system",
+ "sp-api",
+ "sp-core",
+ "sp-runtime",
+ "sp-std",
+]
+
+[[package]]
+name = "bp-message-lane"
+version = "0.1.0"
+dependencies = [
+ "bp-runtime",
+ "frame-support",
+ "frame-system",
+ "parity-scale-codec",
+ "sp-std",
+]
+
+[[package]]
+name = "bp-polkadot"
+version = "0.1.0"
+dependencies = [
+ "bp-message-lane",
+ "bp-runtime",
+ "frame-support",
+ "frame-system",
+ "sp-api",
+ "sp-core",
+ "sp-runtime",
+ "sp-std",
+]
+
+[[package]]
+name = "bp-runtime"
+version = "0.1.0"
+dependencies = [
+ "frame-support",
+ "num-traits",
+ "parity-scale-codec",
+ "sp-core",
+ "sp-io",
+ "sp-runtime",
+ "sp-std",
+]
+
 [[package]]
 name = "bs58"
 version = "0.4.0"