Add contributing section to README
Add contributing section to README

kevinrodbe/minigrid,marcelaraujo/minigrid,msoftware/minigrid,diegochavez/minigrid,JavaStudent/minigrid,Culttm/minigrid,tamdao/minigrid,messinmotion/minigrid,joer19/minigrid,UmarMughal/minigrid,david84/minigrid,henriquea/minigrid,jordan88/minigrid,henriquea/minigrid,chorks/minigrid,chrismoulton/minigrid,bluenaux/minigrid,DigitalCoder/minigrid,digideskio/minigrid,rafaelrinaldi/minigrid,voronianski/minigrid,ls2uper/minigrid