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: 256984997
Test: TH
Change-Id: I62bfeea292050773c9d12cd753357dda836b07b0
diff --git a/target/board/BoardConfigGsiCommon.mk b/target/board/BoardConfigGsiCommon.mk
index 98dea93..4d95b33 100644
--- a/target/board/BoardConfigGsiCommon.mk
+++ b/target/board/BoardConfigGsiCommon.mk
@@ -3,6 +3,8 @@
 # Common compile-time definitions for GSI
 # Builds upon the mainline config.
 #
+# See device/generic/common/README.md for more details.
+#
 
 include build/make/target/board/BoardConfigMainlineCommon.mk