Missing README documentation for signer -> `:routing_order`

[jrpolidario]

Problem

• I needed to find a way to disable "ordering" of multiple signatures, so that signers can sign whenever as they please.

Solution

• Currently, nowhere in README mentions about :routing_order, which I noticed is actually supported already from this particular lines of code
• Therefore, mentioning this in the README would be really helpful.
• I tested that using :routing_order works in my "developer" Docusign account.

Troubleshooting

• (Similar problem where OP is also using docusign_rest gem though he/she didn't answer how he/she solved it) https://support.docusign.com/en/answers/00009385