Change one line in README about building agent. We may eliminate that re...

...quirement later by appending BaseAspect in the generated aj file.
Since this is referring only to the properties that users may write by themselves, this may be confusing for (or give wrong impressions to) users. I think that 

1. Better distinction needs to be made by stating that the user does not need to do anything extra if they use the properties that we already provide.
2. Maybe we should move this to a separate section "Writing your own properties"?