Add Blockchain Reference for the Arweave Namespace

[michaelsena]

Would be great to define a CAIP standard for Arweave. @ceramicnetwork would use this CAIP standard to link an Arweave address to a DID using an account-link document. @samcamwilliams

[pedrouid]

I would be happy to help to write a spec for Arweave cc @samcamwilliams