Update longhorn chart README.md
Update longhorn chart README.md

1. Remove Rancher from prerequisites.
2. Update to Kubernetes v1.15+.
3. Update the contents for installation and uninstallation.

Longhorn #1617

Signed-off-by: Bo Tao <9d7fe1c60cd30b7f3877341fe5713abc4ff31f70@rancher.com>

rancher/longhorn