release.md: Make version bumping process a bit clearer
release.md: Make version bumping process a bit clearer

`make update-version` will not update the `VERSION` file for you, it
needs to be done manually.

Signed-off-by: Krzesimir Nowak <049588669667d6bb20b978e17fc57ff3556ea5dd@kinvolk.io>

kinvolk/habitat-operator,kinvolk/habitat-operator