v0.4.0: adding CHANGELOG

[0.4.0] - 2021-02-17

Added

• CHANGELOG.md
• K8SVAULT_CONTEXT shell variable exported when new shell session is started,
  which can be used in PS1 to display current k8s-vault context
• some spec tests

Changed

• binary path in Docker image is not /bin/k8s-vault
• make version now updates version in example config file
• shard description updated to exclude special symbols
• crystal version set to ~> 0.36.1
• moved argument processing into run method of K8sVault module
• moved helper methods to K8sVault module
• do not display error message when SSH forwarder process is no longer available