asset: warn asset.Open must not be called from init.

Currently there is no easy way to fix golang/go#12077
so we document it.

Fixes golang/go#12077

Change-Id: I6b37ae375673c055152dad5b93e180d9831aef76
Reviewed-on: https://go-review.googlesource.com/13430
Reviewed-by: David Crawshaw <crawshaw@golang.org>

asset/asset.go

@@ -23,6 +23,8 @@ import "io"
 // Open opens a named asset.
 //
 // Errors are of type *os.PathError.
+//
+// This must not be called from init when used in android apps.
 func Open(name string) (File, error) {
 	return openAsset(name)
 }