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/deployments/BridgeDeps.Dockerfile

@@ -0,0 +1,32 @@
+# Image with dependencies required to build projects from the bridge repo.
+#
+# This image is meant to be used as a building block when building images for
+# the various components in the bridge repo, such as nodes and relayers.
+FROM ubuntu:xenial
+
+ENV LAST_DEPS_UPDATE 2020-12-21
+
+RUN set -eux; \
+	apt-get update && \
+	apt-get install -y curl ca-certificates && \
+	apt-get install -y cmake pkg-config libssl-dev git clang libclang-dev
+
+ENV LAST_CERTS_UPDATE 2020-12-21
+
+RUN update-ca-certificates && \
+	curl https://sh.rustup.rs -sSf | sh -s -- -y
+
+ENV PATH="/root/.cargo/bin:${PATH}"
+ENV LAST_RUST_UPDATE 2020-12-21
+
+RUN rustup update stable && \
+	rustup install nightly && \
+	rustup target add wasm32-unknown-unknown --toolchain nightly
+
+RUN rustc -vV && \
+    cargo -V && \
+    gcc -v && \
+    g++ -v && \
+    cmake --version
+
+ENV RUST_BACKTRACE 1