Fix note in devref/contribute.rst
Fix note in devref/contribute.rst

Now note in devref/contribute.rst is formated as a section,
so it will appear at catalog which is pretty ugly.

This fix uses note admonition to replace it and specifies Mitaka
as the release name instead of Mxxx.

Change-Id: Ic4afb086e7d4eace035ebe74d1244eabe1ff8915

javaos74/neutron,watonyweng/neutron,cloudbase/neutron,JianyuWang/neutron,SamYaple/neutron,paninetworks/neutron,dhanunjaya/neutron,sasukeh/neutron,klmitch/neutron,dhanunjaya/neutron,glove747/liberty-neutron,JianyuWang/neutron,suneeth51/neutron,igor-toga/local-snat,wolverineav/neutron,mahak/neutron,cisco-openstack/neutron,takeshineshiro/neutron,bigswitch/neutron,MaximNevrov/neutron,silenci/neutron,apporc/neutron,noironetworks/neutron,mmnelemane/neutron,igor-toga/local-snat,asgard-lab/neutron,dims/neutron,bigswitch/neutron,vivekanand1101/neutron,barnsnake351/neutron,openstack/neutron,openstack/neutron,javaos74/neutron,neoareslinux/neutron,shahbazn/neutron,adelina-t/neutron,jacknjzhou/neutron,eayunstack/neutron,swdream/neutron,cisco-openstack/neutron,MaximNevrov/neutron,paninetworks/neutron,SamYaple/neutron,huntxu/neutron,jumpojoy/neutron,dims/neutron,sebrandon1/neutron,yanheven/neutron,sasukeh/neutron,apporc/neutron,vivekanand1101/neutron,noironetworks/neutron,mattt416/neutron,huntxu/neutron,mahak/neutron,watonyweng/neutron,takeshineshiro/neutron,mattt416/neutron,jacknjzhou/neutron,chitr/neutron,mmnelemane/neutron,mahak/neutron,suneeth51/neutron,glove747/liberty-neutron,jumpojoy/neutron,eayunstack/neutron,barnsnake351/neutron,asgard-lab/neutron,skyddv/neutron,klmitch/neutron,swdream/neutron,yanheven/neutron,silenci/neutron,cloudbase/neutron,sebrandon1/neutron,chitr/neutron,wolverineav/neutron,neoareslinux/neutron,skyddv/neutron,adelina-t/neutron,openstack/neutron,shahbazn/neutron