add a readme.

rules_gathering/README.md

diff --git a/rules_gathering/README.md b/rules_gathering/README.md
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+# Rules and aspects to gather gather package metadata
+
+This folder contains tools used to walk dependency trees and gather
+`LicenseInfo` or similar providers specified by `default_package_metadata`
+and `applicable_licenses`.
+
+
+
+# Known issues:
+
+- [exports_files() is not included](https://github.com/bazelbuild/rules_license/issues/107)