User guide improvements

[ppalaga]

Proposed by our fresh contributor @dhartford in #355 (comment)

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.