update developers guide to include [kind](https://kind.sigs.k8s.io/) (firecracker-microvm#120)

add kind to the list of dependencies in the dev's manual

Signed-off-by: Shyam Jesal <[email protected]>

Author: shyamjesal

docs/developers_guide.md

@@ -41,3 +41,5 @@ Currently, we are in the process of upstreaming VM snapshots support to the upst
 
 * Current Firecracker version is 0.21.0. We plan to keep our code loosely up to date with
 the upstream Firecracker repository.
+
+* vHive uses a [fork](https://github.com/ease-lab/kind) of [kind](https://github.com/kubernetes-sigs/kind) to speed up testing environment setup requiring Kubernetes.