Mention new test matrix in the README

README.rst

@@ -26,18 +26,18 @@
 **Trio-Asyncio** is a re-implementation of the ``asyncio`` mainloop on top of
 Trio.
 
-Trio-Asyncio requires Python 3.6 or 3.7. It should work on Python 3.5.3, but
-that's not tested, as the test suite requires 3.6.
+Trio-Asyncio requires at least Python 3.5.3. It is tested on recent versions of
+3.5, 3.6, 3.7, 3.8, and nightly.
 
 +++++++++++
  Rationale
 +++++++++++
 
 Trio has native concepts of tasks and task cancellation. Asyncio is based
 on callbacks and chaining Futures, albeit with nicer syntax, which make
-handling of failures and timeouts fundamentally less reliable, esp. in
+handling of failures and timeouts fundamentally less reliable, especially in
 larger programs. Thus, you *really* want to base your async project on Trio.
- 
+
 On the other hand, there are quite a few asyncio-enhanced libraries. You
 *really* don't want to re-invent any wheels in your project.