Adds first iteration of 'Why' section to project README

Author: SoftwareEngineerChris

README.md

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 Swift decimals where the number of decimal places is explicitly part of the type
+
+## Why
+
+**_TL/DR: We would like to guarantee a level of precision, or be explicit when changing the level of precision, of decimals moving through our system._**
+
+When dealing with decimals we often want to know; deal with; or maintain a number of decimal places in the number we're representing. 
+
+For example when dealing with money in the simple world, for example in a shopping application, we often want to deal with numbers that have **2 decimal places**. Especially when dealing with lots of these numbers.
+
+If we have a product that costs **$2.50** to sell in the United States, and we wanted to sell it in the United Kingdom, we may want to use an exchange rate to calculate the local price. If the rate provided by our API is **0.81245 USD per GBP** (or **1.23084 GBP per USD**), using a simple multiplication, we may calculate the price for the item to be **£3.0771**.
+
+We probably don't want to present **£3.0771** to our user as they don't normally deal in fractions of pennies. In our presentation layer we may format the value in the receipt as **£3.08**. The user has decided to order **300** of these items. The calculation part of our application doesn't necessarily know how the data is being presented, so deals with the item price of **£3.0771**. Therefore, **£3.0771** multiplied by **300** is **£923.13**.
+
+In this case we may present a receipt to the user which looks like: `Special Product @ £3.08 x 300 = £923.13`
+
+Except **300** multiplied by **£3.08** _isn't_ **£923.13**. It's **£924.00**. Our user is confused about the total we're presenting to them, not realising that our calculation used a price of £3.0771 per item rather than the £3.08 presented to them. Not only that, but we're sending the presented item price to our analytics system - and its not aligning with the totals in our sales system. What a nightmare.
+
+What they should have seen on their receipt is either:
+`Special Product @ £3.08 x 300 = £924.00` or  `Special Product @ £3.0771 x 300 = £923.13`
+
+Of course, how this is handled should be a business decision, but we should be able to guarantee the decision in our code. If we can leverage the type system to help us out, we can make this a compile-time requirement. I hope that RoundedDecimal can help us with that.