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/.editorconfig

@@ -0,0 +1,16 @@
+root = true
+[*]
+indent_style=tab
+indent_size=tab
+tab_width=4
+end_of_line=lf
+charset=utf-8
+trim_trailing_whitespace=true
+max_line_length=100
+insert_final_newline=true
+
+[*.{yml,md,yaml,sh}]
+indent_style=space
+indent_size=2
+tab_width=8
+end_of_line=lf