Readme.md: add badges
Readme.md: add badges

Change-Id: I8280b7b7f4239dd4f481bd7a30d8f0ec6d5928e4
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2290
Reviewed-by: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@google.com>

cuelang/cue