Add more information on the "void" methods

[ejdaly]

voidInvoice() and voidPayment() are not documented yet:
1a5a62a

There's also some useful info on voiding in this issue, that would be useful in the main readme:
#43

Also, it seems "void" is an acceptable operation to provide to Batch Operations (which is useful..., but not documented).

For voiding payments using Batch Operations, it seems you can't use the "void" operation however. Instead you need to specify:

operation: "update"
optionsData: "void"

https://community-intuit.force.com/developer/s/question/0D50f00005E4WI6/how-do-i-void-payments-through-the-batch-endpoint-of-the-api

(I'm not sure if you want to handle that case within the library, or just document it..)

If I get time in the coming weeks - I'll try put together a PR for these documentation updates ... but at least they are visible here in the meantime.

[ShrihariMohan]

#202