Add Credit Card Documentation (#419)

Add documentation on how to fix carrying over debt from credit cards
when importing from nYNAB.

docs/migration/nynab.md

@@ -54,6 +54,21 @@ curl -H "Authorization: Bearer <ACCESS_TOKEN>" https://api.youneedabudget.com/v1
 - Choose the exported json file
 
 ### Optional: Cleanup
+
+#### Credit Cards (Fix Overspending)
+
+If you import credit cards with previous debt, you must handle these differently. Otherwise, your budget months will show overspending. Actual does not handle carrying over debt the same way, but offers a more manual approach.
+
+1. From the Budget screen, create a category named `Credit Card` (perhaps under a Category Group of `Debt`).
+2. Change all overspent transactions to have their category be this `Credit Card` category.
+3. On the first month of overspending for this category, click on the Balance (it should show red) and select `Rollover overspending`.
+4. Next, you must assign money each historical month to cover any payments of the `Credit Card` category. Open YNAB and look through each month to find the "extra" amount assigned to the card. (To find the "extra," open YNAB and look through each month. Find the amount assigned to the card in excess of any additional monthly spending, which is how much was used to pay the debt on the credit card.) Put this amount in your `Credit Card` category in Actual.
+5. If your budget isn't zeroing out yet, follow the instructions below in _Hold For Next Month_.
+
+A full description of how to carry over debt can be found in [our Carrying Debt article.](https://actualbudget.org/docs/budgeting/credit-cards/carrying-debt)
+
+#### Hold for Next Month (Fix Money Leftover in To Budget)
+
 nYNAB calculates its `Ready to Assign` value differently than Actual's `To Budget` value.
 There is no need to worry, we can make them match exactly with a simple change.
 This is purely a visual change and doesn't affect the budget itself.