Add Exchange FAQ document on the wiki

[lehnberg]

https://github.com/mimblewimble/grin-pm/blob/master/notes/20181227-meeting-development.md#8-other-questions:

lehnberg: I've had loads of questions from an exchange. Look similar to the questions that another exchange was asking. So I’m putting together a big FAQ doc that I’m hoping the community will help populate, and we keep on the wiki. That we then can post exchanges to, and help them help themselves.

[ghost]

https://github.com/mimblewimble/site/issues/57 Maybe relevant here also

[0xmichalis]

Can be closed now with https://github.com/mimblewimble/docs/wiki/Exchange-FAQ?