Parachain documentation

[nuke-web3]

I am opening this issue to gather insights into what needs to be added to our devhub as it relates to parachains.
Please let's use this to discuss what we think is needed, and priorities for this.

---

There is a general lack of information on the devhub as it relates to cumulus, parachains, and the nuances that make it different thatn solo-chain development with substrate.

A few topics that I see as critically needed:

• collator selection (not using BABE, what options are there?)
• reading data from the relay chain state, like randomness on chain (related: Support custom storage proof from the relay-chain paritytech/polkadot-sdk#82)
• XCM & XCMP (plus examples)
• Bridging (https://github.com/paritytech/parity-bridges-common/) and compare/contrast with XCMP
  • workshop: https://www.youtube.com/watch?v=rrpDBvac7gI
• ... I am sure there is more.

What do you think should be added, and what takes priority?

[nuke-web3]

polkadot-developers/substrate-docs#179