Add integration docs to the repo

[hackfisher]

https://itering.notion.site/How-to-Develop-a-Substrate-to-Substrate-Bridge-98b50a6e96774c1984d606a549f0a550

[hackfisher]

Add the convention of dependency management for bridges and (third party) chains to follow.

[hackfisher]

https://github.com/darwinia-network/darwinia-bridges-substrate/tree/main/bin/runtime-common

[wuminzhe]

I will try to build a bridge from scratch. Any suggestion?

[boundless-forest]

On the node side, you can run a pangolin <> pangoro network locally and figure out the configuration of those two chains' runtime(bridges-grandpa, bridges-messages, bridges-dispatch, bridges-fee-market), here is a script https://github.com/darwinia-netwrk/darwinia-common#development to help you set up node easier. In addition, you also need to run a bridger, follow the README and have a try.