add ballvalve to the list of prior art (#127)

Might as well, since it looks like the spec will actually apply to AsyncIterator also.

Author: robey

README.md

@@ -80,6 +80,7 @@ any form of iterator, different iterators have to be handled differently.
 - https://www.boost.org/doc/libs/1_66_0/libs/iterator/doc/index.html
 - https://docs.microsoft.com/en-us/dotnet/api/system.linq.enumerable
 - https://github.com/ReactiveX/IxJS
+- https://www.npmjs.com/package/ballvalve
 
 | Method                      | Rust | Python | npm Itertools | C# |
 | --------------------------- | ---- | ------ | --------------| -- |