add scorecard badge to README file
[![Godoc](http://img.shields.io/badge/go-documentation-blue.svg?style=flat-square)](https://godoc.org/github.com/etcd-io/etcd)
[![Releases](https://img.shields.io/github/release/etcd-io/etcd/all.svg?style=flat-square)](https://github.com/etcd-io/etcd/releases)
[![LICENSE](https://img.shields.io/github/license/etcd-io/etcd.svg?style=flat-square)](https://github.com/etcd-io/etcd/blob/main/LICENSE)
+[![OpenSSF Scorecard](https://api.securityscorecards.dev/projects/github.com/etcd-io/etcd/badge)](https://api.securityscorecards.dev/projects/github.com/etcd-io/etcd)
**Note**: The `main` branch may be in an *unstable or even broken state* during development. For stable versions, see [releases][github-release].
