[docs] Add XCM / XCMP article

[sacha-l]

Add an article for parachain builders. Work in progress.

[nuke-web3]

<@olanod:virto.community> Is there some source to read more information about the barriers, queries&subscriptions and other newer XCM related topics?

https://polkadot.network/blog/xcm-part-three-execution-and-error-management/

https://github.com/paritytech/xcm-format

all three parts are a good start. subscribe to the format repo for any changes. More docs soon on this.

[nuke-web3]

Add info on using public good parachains. Tracking:

• [ ] https://github.com/paritytech/polkadot-sdk/issues/107

[joepetrowski]

Add info on using public good parachains.

I wouldn't quite characterize it in that way. It's reserve backed transfer of assets via the chain acting as reserve. The reserve chain of a particular asset may or may not be a common good chain. Likewise, other common good chains could send cross-chain messages that are not asset-related (although may use some asset to buy execution in a block).

[bernardoaraujor]

any estimations on when will this be available?

[sacha-l]

I think the best we can do is have a page that explains basic XCM principles and how to use them, and link to all other current resources on XCM. For more advanced things, we could maintain a Q&A list - what do you think @NukeManDan @shawntabrizi ?

[shawntabrizi]

Truth is that XCM is still too much in development for us to write any kind of deep technical documentation. I understand that parachain builders want more docs, but practically speaking, I dont think we have the capacity or ability to do this well.

Instead, we should point users to the source code, numerous tests, and high level documentation which explains concepts which are unlikely to change from xcm version to version.

[nuke-web3]

stub is included in paritytech/cumulus#1140 , still at this time not in a place I would consider user facing

[bernardoaraujor]

this Audit Report on XCMv2 could be a really good reference for XCM documentation.