Charging items to clients - Xero process

[rub1e]

Items like Heroku hosting which are charged to a client also need to be tracked as part of a project in Xero - so they're included when we run reports

So I've added a tracking code - 888 - for ongoing items which aren't part of a sprint.

(This is more of a reminder to me to put this into the appropriate md file than anything else)