Link to readme from ci.bzl

Summary: It is not obvious that readme exists when opening `ci.bzl` while trying to understand what it does.

Reviewed By: ndmitchell

Differential Revision: D59635090

fbshipit-source-id: 9bd4a5bbdc8be7443208e3c909595d14a044b70e

Author: stepancheg

shim/target_determinator/macros/ci.bzl

@@ -5,6 +5,8 @@
 # License, Version 2.0 found in the LICENSE-APACHE file in the root directory
 # of this source tree.
 
+# https://internalfb.com/code/fbsource/fbcode/target_determinator/macros/README.md
+
 def _lbl(*_args):
     return ""