Add docs/blurb about accuracy against Snowflake bill

[ian-whitestone]

## Accuracy relative to your Snowflake bill

The `daily_spend` model has been reconciled against multiple Snowflake accounts and lines up exactly (with the exception of [some rounding errors](https://github.com/get-select/dbt-snowflake-monitoring/issues/25), to be fixed in an upcoming release).

If you'd like to validate it for yourself, you can run the following queries:

```sql
-- model
select
    date,
    service,
    sum(spend) as spend
from daily_spend
group by 1,2
order by date desc, spend desc
;

-- actual
select
    usage_date as date,
    usage_type as service,
    usage_in_currency as spend
from snowflake.organization_usage.usage_in_currency_daily
where account_locator='<your_account_locator>' -- Not required if you only have 1 account
order by date desc, spend desc
;