Add documentation on how to sign Michelson data

[roxaneletourneau]

Add examples/explanations on how to sign Michelson data.

One scenario example will be to start with raw Michelson data, parse it to Michelson JSON, pack it, and then sign:

const data = `(Pair (Pair { Elt 1
                  (Pair (Pair "tz1gjaF81ZRRvdzjobyfVNsAeSC6PScjfQwN" "tz1KqTpEZ7Yob7QbPE4Hy4Wo8fHG8LhKxZSx")
                        0x0501000000026869) }
            10000000)
      (Pair 2 333))`;
const type = `(pair (pair (map int (pair (pair address address) bytes)) int) (pair int int))`;
const p = new Parser()
const dataJSON  = p.parseMichelineExpression(data)
const typeJSON = p.parseMichelineExpression(type)
const pack = await tezos.rpc.packData({ data: dataJSON as MichelsonV1Expression, type: typeJSON as MichelsonV1Expression });
const sign = await tezos.signer.sign(pack.packed);