Readme.md: add GolangCI badge

Change-Id: I210953040a10438f1eaf030461ea46ae206b5b4e
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2951
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/README.md b/README.md
index f23c48f..b33ec3c 100644
--- a/README.md
+++ b/README.md
@@ -16,6 +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)
+[![GolangCI](https://golangci.com/badges/github.com/cuelang/cue.svg)](https://golangci.com/r/github.com/cuelang/cue)
 [![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)]()