Add a textual hint as to where aidl has gone

Bug: 23691099
Change-Id: I3fb917b4c0e66a83c6299add02869a7088010adc
diff --git a/tools/aidl b/tools/aidl
new file mode 100644
index 0000000..8a42fa0
--- /dev/null
+++ b/tools/aidl
@@ -0,0 +1,4 @@
+Where has aidl gone?
+
+  aidl has moved to //system/tools/aidl as part of adding support for
+  generating bindings in C++.