Add notice about 3.0.1

Strictly speaking bumping to 4.0.0 will adhere more strongly to semver, but indeed we've broken semver before in 2.x and this breakage is very unlikely to affect anyone.

Thoughts?

docs/docs/new-in-bluebird-3.md

@@ -41,7 +41,6 @@ Warnings have been added to report usages which are very likely to be programmer
 - Added [.catchReturn\(\)](.).
 - Added [Promise.mapSeries\(\)](.) and [.mapSeries\(\)](.)
 
-
 ##Deprecations
 
 - `Promise.settle` has been deprecated. Use [.reflect](.) instead.
@@ -59,3 +58,7 @@ Warnings have been added to report usages which are very likely to be programmer
 - [.spread](.)'s second argument has been removed.
 - [.done](.) causes an irrecoverable fatal error in Node.js environments now. See [#471](.) for rationale.
 - Errors created with [Promise.reject](.) or `reject` callback of [new Promise](.) are no longer marked as [OperationalError](.)s.
+
+##3.0.1 update
+
+Note that the 3.0.1 update is strictly speaking backward-incompatible with 3.0.0. Version 3.0.0 broke the previous behavior of the `.each` method and made it work more like the new `.mapSeries` - 3.0.1 fixes this breakage by reverting to the `.tap`-like behavior found in 2.x However, this would only affect users who updated to 3.0.0 during the short time that it wasnt deprecated and started relying on the new `.each` behavior. This seems unlikely, and therefore the major version was not changed.