add out of scope: currencies to section 2

add out of scope currencies and why (per [comment from mt](#23 (comment))), with extensibility considerations, and possibly numerical-punctuation only support

amount.md

@@ -15,4 +15,7 @@ Per the priority of constituencies, we should first look for solutions that user
 e.g. if a solution enables an affordance to convert units, it should be 100% reliable like a calculator, 
 as users will expect any unit conversions to be simple arithmetic.
 Second, we should look for solutions that are minimal work, simpler, and more dependable and robust for developers. 
-In particular, we should look for and prefer [declarative approaches](https://www.mozilla.org/en-US/about/webvision/full/#thedeclarativeweb) over imperative approaches.
+In particular, we should look for and prefer [declarative approaches](https://www.mozilla.org/en-US/about/webvision/full/#thedeclarativeweb) over imperative approaches. 
+
+Out of scope: 
+* currencies (for this Explainer), because their conversions are subjective and changing (not 100% reliable). We may still consider the backward compatible extensibility of any solutions, in order to enable a future Explainer to add currency support. We may also consider how solutions could treat a currency amount as a numerical amount with an unknown unit, for the purposes of converting at least the numerical portion to user preferred or locale-specific punctuation for large or decimal numbers.