Caveat about async in create-subscription README (facebook#12469)

* Caveat about async in create-subscription README

* Address Sophie's comments

* Dan's nits

packages/create-subscription/README.md

@@ -11,6 +11,16 @@ Other cases have **better long-term solutions**:
 * I/O subscriptions (e.g. notifications) that update infrequently should use [`simple-cache-provider`](https://github.com/facebook/react/blob/master/packages/simple-cache-provider/README.md) instead.
 * Complex libraries like Relay/Apollo should manage subscriptions manually with the same techniques which this library uses under the hood (as referenced [here](https://gist.github.com/bvaughn/d569177d70b50b58bff69c3c4a5353f3)) in a way that is most optimized for their library usage.
 
+## Limitations in async mode
+
+The main motivation for `create-subscription` is to provide a way for library authors to ensure compatibility with React's upcoming asynchronous rendering mode. `create-subscription` guarantees correctness in async mode, accounting for the subtle bugs and edge cases that a library author might otherwise miss.
+
+However, it achieves correctness by sometimes de-opting to synchronous mode, obviating the benefits of async rendering. This is an inherent limitation of storing state outside of React's managed state queue and rendering in response to a change event.
+
+The effect of de-opting to sync mode is that the main thread may periodically be blocked (in the case of CPU-bound work), and placeholders may appear earlier than desired (in the case of IO-bound work).
+
+For **full compatibility** with asynchronous rendering, including both **time-slicing** and **React Suspense**, the suggested longer term solution is to move to one of the patterns described in the previous section.
+
 ## What types of subscriptions can this support?
 
 This abstraction can handle a variety of subscription types, including: