Added a line of documentation to the ReadMe for using the mixpanel_people_set method.

I thought it'd be helpful to add this line to the ReadMe doc. It took me a little bit to work out the syntax for the mixpanel_people_set method since several of the properties require $. For example: 

```
mixpanel_people_set( '$name' => "#{current_user.name}", '$email' => "#{current_user.email}")
```