readme: add build status badge

Signed-off-by: Samuel Karp <[email protected]>

Author: samuelkarp

README.md

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 A basic Go interface to the Firecracker API
 ====
 
+[![Build status](https://badge.buildkite.com/de08ca676829bedbf6de040c2c2ba1a5d2892e220997c2abdd.svg)](https://buildkite.com/firecracker-microvm/firecracker-go-sdk)
+
 This package is a Go library to interact with the Firecracker API. It
 is designed as an abstraction of the OpenAPI-generated client that
 allows for convenient manipulation of Firecracker VM from Go programs.