Update FAQ.md (#1186)

FAQ.md

@@ -68,6 +68,14 @@ Some number ranges are explicitly defined as being for fixed-line or mobile
 phones. We even represent ranges defined as being "Mostly land-line" in this
 way.
 
+### What is mobile number portability?
+
+The ability to keep your mobile phone number when changing carriers. To see whether a region supports mobile number portability use [isMobileNumberPortableRegion](https://github.com/googlei18n/libphonenumber/blob/master/java/libphonenumber/src/com/google/i18n/phonenumbers/PhoneNumberUtil.java#L3275).
+
+### Since it's possible to change the carrier for a phone number, how is the data kept up-to-date?
+
+Not all regions support mobile number portability. For those that don't, we return the carrier when available. For those that do, we return the original carrier for the supplied number.
+
 ### What about M2M (machine to machine) numbers?
 
 libphonenumber does not support M2M numbers at the moment, but might in the