Add an option to specify category in generated user.properties

[belong-events]

The problem

I don't want to add category to each source item because they are all the same per environment.
It is clunky to specify different value according to each environment.

The cause

There is no way to specify category for auto-generated user.properties.

The goal

Provide an option to add category to auto-generated user.properties.

The solution

• Enable this by an environment variable as other settings (in run.sh)
• Update the README