Write up bank reconciliation process

### Why?
In order to pass all of our accounts onto our accountants for auditing/year end filings or anything else that is required, we need to first reconcile our accounts with the bank feeds so that they know we can account for each transaction.

### What
Bank reconciliations are a charming world unto themselves.

This is the act of matching each transaction from our bank account with the relevant receipt, bill or invoice, filing them into the correct category. This then allows our accountants to go through the accounts and determine what is tax deductible (i.e. counts as a business expense) and what is not ('client entertainment' for example, does not count as a business expense), allowing them to then calculate how much corporation tax we need to pay at the end of the year.

We don't have this documented yet but it is a necessary process, so we must do so.

Making this a `priority-2` as we are currently finalising our reconciliations and this is an effective time to write this up!
Moving this here from https://github.com/dwyl/hq/issues/414

* [x] Write up Coop bank feed update process (#127)
* [x] Write up Santander feed update process (#129)
* [ ] Write up full reconciliation process for Xero (#120)