Attempt #2 to unify the README files. (#48)

* Merge /README.md into /charts/docker-mailserver/README.md to reduce duplication.

(Attempt #2)

* readme: Clarify documentation around helm chart location and installation.

Fixes #44.

* Bump chart version.

Co-authored-by: David Young <davidy@funkypenguin.co.nz>

Author: rspier