add FAQ to README

README.md

@@ -60,6 +60,63 @@ http://godoc.org/github.com/shopspring/decimal
 * [Spring](https://shopspring.com/), since August 14, 2014.
 * If you are using this in production, please let us know!
 
+## FAQ
+
+#### Why don't you just use float64?
+
+Because float64s (or any binary floating point type, actually) can't represent
+numbers such as 0.1 exactly.
+
+Consider this code: http://play.golang.org/p/TQBd4yJe6B You might expect that
+it prints out `10`, but it actually prints `9.999999999999831`. Over time,
+these small errors can really add up!
+
+#### Why don't you just use big.Rat?
+
+big.Rat is fine for representing rational numbers, but Decimal is better for
+representing money. Why? Here's a (contrived) example:
+
+Let's say you use big.Rat, and you have two numbers, x and y, both
+representing 1/3, and you have `z = 1 - x - y = 1/3`. If you print each one
+out, the string output has to stop somewhere (let's say it stops at 3 decimal
+digits, for simplicity), so you'll get 0.333, 0.333, and 0.333. But where did
+the other 0.001 go?
+
+Here's the above example as code: http://play.golang.org/p/lCZZs0w9KE
+
+With Decimal, the strings being printed out represent the number exactly. So,
+if you have `x = y = 1/3` (with precision 3), they will actually be equal to
+0.333, and when you do `z = 1 - x - y`, `z` will be equal to .334. No money is
+unaccounted for!
+
+You still have to be careful. If you want to split a number `N` 3 ways, you
+can't just send `N/3` to three different people. You have to pick one to send
+`N - (2/3*N)` to. That person will receive the fraction of a penny remainder.
+
+But, it is much easier to be careful with Decimal than with big.Rat.
+
+#### Why isn't the API similar to big.Int's?
+
+big.Int's API is built to reduce the number of memory allocations for maximal
+performance. This makes sense for its use-case, but the trade-off is that the
+API is awkward and easy to misuse.
+
+For example, to add two big.Ints, you do: `z := new(big.Int).Add(x, y)`. A
+developer unfamiliar with this API might try to do `z := a.Add(a, b)`. This
+modifies `a` and sets `z` as an alias for `a`, which they might not expect. It
+also modifies any other aliases to `a`.
+
+Here's an example of the subtle bugs you can introduce with big.Int's API:
+https://play.golang.org/p/x2R_78pa8r
+
+In contrast, it's difficult to make such mistakes with decimal. Decimals
+behave like other go numbers types: even though `a = b` will not deep copy
+`b` into `a`, it is impossible to modify a Decimal, since all Decimal methods
+return new Decimals and do not modify the originals. The downside is that
+this causes extra allocations, so Decimal is less performant.  My assumption
+is that if you're using Decimals, you probably care more about correctness
+than performance.
+
 ## License
 
 The MIT License (MIT)