Document Atest support for wildcards in ART run-tests invocations.

Mention that Atest supports wildcards in test names and how this can
be used to run ART run-tests in `art/test/README.atest.md`.

Test: Render `README.atest.md` as HTML and check the output manually
Bug: 147818031
Change-Id: I0b9771a4b9685b2cb27cd95a0feeba1d22ff430b
diff --git a/test/README.atest.md b/test/README.atest.md
index d71c649..dc3878a 100644
--- a/test/README.atest.md
+++ b/test/README.atest.md
@@ -30,6 +30,16 @@
 To run all ART run-tests in a single command, the currently recommended way is
 to use [test mapping](#test-mapping) (see below).
 
+You can nonetheless run all supported ART run-tests with a single Atest command,
+using its support for wildcards:
+```bash
+atest art-run-test-\*
+```
+
+Note: Many ART run-tests are failing with the TradeFed harness as of March 2021,
+so the above Atest command will likely report many tests failures. The ART team
+is actively working on this issue.
+
 ## ART gtests
 
 ### Running ART gtests on device