design/19308-number-literals: add self-link

Change-Id: Ib56c8b7aacf2b5c22c63fe6a654f1cf68871e8d7
Reviewed-on: https://go-review.googlesource.com/c/158240
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
diff --git a/design/19308-number-literals.md b/design/19308-number-literals.md
index 585c498..bb53afd 100644
--- a/design/19308-number-literals.md
+++ b/design/19308-number-literals.md
@@ -5,6 +5,8 @@
 
 Last updated: January 16, 2019
 
+[golang.org/design/19308-number-literals](https://golang.org/design/19308-number-literals)
+
 Discussion at:
 
    - [golang.org/issue/19308](https://golang.org/issue/19308) (binary integer literals)