INSTALL.md: Clarify requirements for namespaced installs

[liviuchircu]

Hi!

We twice (on different K8s clusters) ran into the same issue with a Helm-based, namespaced Operator install where the CPA CRD would never create the autoscaler Pod, and nothing useful would be logged. After some trial and error, we discovered that we also needed to install the Operator into the default namespace, together with the namespaced install. Once both installs were in place, the autoscaler Pod magically appeared and started doing its nice work.

Making this small PR so others don't run into the same issue. Or, if you have any suggestions, please advise!

Thank you for the amazing project! It's the least we can give back! :)

[jthomperoo]

Thanks very much for pointing this out and providing the docs update! So on a little bit of further investigation it looks like you've found a bug 😱

I've raised this issue to address this #89

Thanks again!