User guide improvements

Proposed by our fresh contributor @dhartford in https://github.com/apache/camel-quarkus/pull/355#issuecomment-548427942

> 1. Reference how to safely git rebase/squash commits before a PR. There are a lot of references on the internet, however not all are made equal as I found out.
> 2. A starter-guide for how to identify classes that need special reflection handling for GraalVM/native builds. I know that isn't necessarily camel-specific, but likely to have a lot of experienced java developers that want to contribute that will be in the dark with that item.

Start with the two requested topics in the issue body and read the referenced pull-request comment for context. Done means adding guidance on safely rebasing or squashing commits and a starter guide for identifying classes needing special reflection handling in GraalVM/native builds.

Written by the indexing model from the issue text.