fs: add package comment

Even though the package is a work in progress, it's still helpful
for it to have a description.
dmitshur committed 2 years ago commit 266637f44bda0dc8f1cf767182c7d7e5b3763725
@@ -1,5 +1,8 @@
+// Package fs will implement change.Service using a virtual filesystem,
+// once change.Service API is finalized. Until then, it uses hardcoded
+// mock data to aid development and evaluation of change.Service API.
 package fs
 import (