Update README.md

[codrinh]

Hello,

Since the latest release 2.* of armory-observability-plugin our spinnaker setup failed to start.
We had to pin the previous version of the plugin to make it work again.

Please update the documentation on how to use a specific version of the plugin.

    spinnaker:
      extensibility:
        plugins:
          Armory.ObservabilityPlugin:
            enabled: true
            version: 1.6.0

Thank you.

[iniinikoski]

Definitely - same here. Good finding @codrinh. Ping @jasonmcintosh :)