Mark xaac codec experimental

Bug: 117786798
Test: presence in source tree
Change-Id: I89475f9b8b09b22d6924ae8e97275206c2d85be8
(cherry picked from commit e5f65556cc592d9faa6a225223cf7697b2880b7b)
diff --git a/README.experimental b/README.experimental
new file mode 100644
index 0000000..27fd798
--- /dev/null
+++ b/README.experimental
@@ -0,0 +1,5 @@
+This xaac codec (external/xaac) is experimental; it is not yet intended
+to be used on production devices.
+
+This codec should not be configured into any production Android Pie
+(Android 9) device that will be shipped.