updated README breaking add_cpd into four different functions
updated README breaking add_cpd into four different functions

yashu-seth/pgmpy,liquidmetal/pgmpy,pgmpy/pgmpy,abinashpanda/pgmpy,sandeepkrjha/pgmpy,kislayabhi/pgmpy,sandeepkrjha/pgmpy,pratyakshs/pgmpy,khalibartan/pgmpy,liquidmetal/pgmpy,pgmpy/pgmpy,kris-singh/pgmpy,vivek425ster/pgmpy,ankurankan/pgmpy,ankurankan/pgmpy,kris-singh/pgmpy,jhonatanoliveira/pgmpy,jhonatanoliveira/pgmpy,yashu-seth/pgmpy,palashahuja/pgmpy,vivek425ster/pgmpy,anaviltripathi/pgmpy,khalibartan/pgmpy,palashahuja/pgmpy,pratyakshs/pgmpy,anaviltripathi/pgmpy,abinashpanda/pgmpy,kislayabhi/pgmpy