Provide documentation for upgrade containing breaking changes

As already noted in some issue reports, sometimes there are breaking changes which require special attention to how a smooth upgrade is being approached.

Some scenarios include:

1. upgrading to a new "major" version of OpenObserve, e.g. 0.9.x → 0.10.x (see Issue when upgrading from 0.9.1 to 0.10.1 #57 )
2. breaking changes made to the helm chart itself, e.g. changing the service name (see Use headless service for statefulSet serviceName #65 )

Release notes for helm charts would also help to indicate whether a new chart version requires special care or not.

[emerconn]

+1 for Helm chart release notes, please 😄

These are very convenient for Renovate PRs on Flux repos, like this example.