Readme.md: add GolangCI badge
Readme.md: add GolangCI badge

Change-Id: I210953040a10438f1eaf030461ea46ae206b5b4e
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2951
Reviewed-by: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@golang.org>

cuelang/cue