Add the path of GSI document in gsi_release.mk

Documents for GSI specific packages can be found at
device/generic/common/README.md.

Bug: 254392480
Test: Presubmit
Change-Id: I03e9d66462f39c0631dfcbe16902e5978e41c8b6
diff --git a/target/product/gsi_release.mk b/target/product/gsi_release.mk
index 20493be..3705a50 100644
--- a/target/product/gsi_release.mk
+++ b/target/product/gsi_release.mk
@@ -23,6 +23,8 @@
 # - Released GSI contains more VNDK packages to support old version vendors
 # - etc.
 #
+# See device/generic/common/README.md for more details.
+#
 
 BUILDING_GSI := true