normally, I would try to find the math formula that answers this
question. however, as this is a simple programming exercise, a while
loop it is, as given by the instructor.

this is literally verbatim copy from the course

* added a note on python division and integer division
* also described the pupose of learnings.md in it.

* also made the `extra_payment_end_month` inclusive. In the previous
exercise this statement was given:

> How much will Dave pay if he pays an extra $1000/month for 4 years starting in year 5 of the mortgage?

along with a code snippet

```
extra_payment_start_month = 60
extra_payment_end_month = 108
```

From these it's clear the start month is inclusive and end month is
exclusive. However, made the end month inclusive to match the result
shown in the course in this exercise.