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/.gitignore

@@ -0,0 +1,22 @@
+**/target/
+**/.env
+**/.env2
+**/rust-toolchain
+
+**/*.rs.bk
+
+*.o
+*.so
+*.rlib
+*.dll
+.gdb_history
+
+*.exe
+
+.DS_Store
+
+.idea
+.vscode
+*.iml
+*.swp
+*.swo