Add .ruby-version to use specific version

[philonor]

Description
Commit .ruby-version file to lock a specific ruby version.

Reason
On MacOS Catalina, ruby 2.6. is shipped by default and rbenv (brew install rbenv) defaults to 2.6.
With this configuration, installing gem nokogiri fails.
By both locking the nvm environment with .nvmrc and ruby environment with .ruby-version, it is ensured that users of this repository have comparable environments and both bug reporting and fixing becomes easier.

Corresponding Wiki Changes
Branch: https://github.com/philonor/mailmason-wiki/tree/hf/setup-rbenv
Changeset: https://github.com/philonor/mailmason-wiki/compare/43d7da4..002ab3d
Changes:

• Add information about ruby version / rbenv
• Split setup in preliminaries and actual setup
• Add information for sample config JSON files to avoid Required config property "watch" missing #36