1. Clarify that you need to manually create the user and group
when building from source. ./build.sh does not do that since
it is a packaging tool, not system configuration one.
2. Do not require "go" command to be present when running
./build.sh install. go installation may be user-specific and
unavailable when running with sudo.
3. Ease UMask restrictions. Allow group access.
This allows CLI commands to be run by any user in maddy group.
See #569.