design/draft-fuzzing.md: describe `go clean -fuzzcache`

Change-Id: Iaec8b8c9ee62bd348ec0bb7dd5303c859b0652de
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/295490
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
diff --git a/design/draft-fuzzing.md b/design/draft-fuzzing.md
index fe41717..19bf32f 100644
--- a/design/draft-fuzzing.md
+++ b/design/draft-fuzzing.md
@@ -480,6 +480,12 @@
 `go test` will not respect `-p` when running with `-fuzz`, as it doesn't make
 sense to fuzz multiple packages at the same time.
 
+There will also be a new flag, `-fuzzcache` introduced to `go clean`.
+When this flag is not set, `go clean` will not automatically remove generated
+corpus files, even though they are written into a subdirectory of `$GOCACHE`.
+In order to remove the generated corpus files, one must run
+`go clean -fuzzcache`, which will remove all generated corpus in `$GOCACHE`.
+
 ## Open questions and future work
 
 ### Fuzzing engine supports multiple targets at once