Document workflow for signing multisig transactions

[zmanian]

There is no documentation on the workflow for using multisig in the gaiacli to sign and brodcast a multisig transaction.

The send tokens example in https://github.com/cosmos/cosmos-sdk/blob/develop/docs/gaia/gaiacli.md only covers the single sig case.

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[alexanderbez]

Thanks @zmanian, you are indeed correct it seems. I'll be happy to update the docs.

[alexanderbez]

Actually, afaik, we only have functionality to generate a multisig address/pubkey pair, but not actually having the ability to sign & broadcast txs with a multisig. (cc @alessio).

[alessio]

Will pick this up.