pubsub multibase encoding

[TMoMoreau]

Adds clarification for pubsub multibase encoding over HTTP RPC for issue ipfs/ipfs-docs#1007

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[TheDiscordian]

CC @johnnymatthews (you asked to be pinged about this once the PR is up once upon a time)

[johnnymatthews]

Lovely stuff. Just need a maintainer of this repo to merge and we're good to go!