Update documentation about how envman stores data

[hisaac]

@DamienBitrise found what looks like some outdated documentation.

Outdated Documentation

• envman does not store its data in a file named .envstore as the documentation currently states. It stores it in a file named .envstore.yml.
• Running envman add doesn't create the .envstore.yml file as the documentation currently states. The user must run envman init to create an empty .envstore.yml file first, prior to adding values.

Changes

I've updated the documentation to reflect how envman works today.