Add CI status shield in README
Summary:
Pretty standard for open source projects
Pull Request resolved:
# resctl
+![CI](https://github.com/facebookincubator/resctl/workflows/CI/badge.svg)
+![Matrix chat](https://img.shields.io/matrix/below:matrix.org)
+
resctl is an umbrella repository for **res**ource **c**on**t**ro**l** projects
at Facebook.
