Update Install instructions

Update Install instructions:

- [x] finish the code
- [x] gather feedback for my changes
- [x] submit changes to the documentation


| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| Tests pass?   | none
| Fixed tickets | none
| Doc           | https://github.com/sulu-cmf/sulu-standard/blob/feature/install-update/README.md#download-and-install-jackrabbit