You got the first part right.. now

you need actual file for your code, let's call it hr_salary.py . 
next, you need to add import hr_salary  in __init__.py of your module 

Now you are ready to inherit the class hr_employee, in hr_salary.py start with:

class hr_salary_rule(osv.Model):
     _inherit = "hr.salary.rule"
     # the rest of your class code goes here

After you have done coding, save your file, and restart openerp service...
 

hope it helps...