Adopt CII Best Practices Badge (#1726)
Adopt CII Best Practices Badge (#1726)

Required for CNCF graduation.

Completed CII Best Practices
(https://bestpractices.coreinfrastructure.org/projects/1445), adding
badge to README.md.

Fixes #963

Signed-off-by: Andrew Seigner <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@sig.gy>
linkerd/linkerd,BuoyantIO/linkerd,linkerd/linkerd,linkerd/linkerd,BuoyantIO/linkerd