README.md: bump supported go version to 1.12+

Change-Id: I8cee9dec3ec13dceb19945135d078444f02a10e4
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/3862
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/README.md b/README.md
index e67c6c0..f23c48f 100644
--- a/README.md
+++ b/README.md
@@ -16,7 +16,7 @@
 [![GoDoc](https://godoc.org/cuelang.org/go?status.svg)](https://godoc.org/cuelang.org/go)
 [![Github](https://github.com/cuelang/cue/workflows/Test/badge.svg)](https://github.com/cuelang/cue/actions)
 [![Go Report Card](https://goreportcard.com/badge/github.com/cuelang/cue)](https://goreportcard.com/report/github.com/cuelang/cue)
-[![Go 1.12](https://img.shields.io/badge/go-1.12-9cf.svg)](https://golang.org/dl/)
+[![Go 1.12+](https://img.shields.io/badge/go-1.12-9cf.svg)](https://golang.org/dl/)
 [![platforms](https://img.shields.io/badge/platforms-linux|windows|macos-inactive.svg)]()