keystone-auth.md: Add note about api compatibility
keystone-auth.md: Add note about api compatibility

meng-sun/hil,henn/hil_sahil,henn/haas,henn/hil_sahil,kylehogan/hil,meng-sun/hil,henn/hil,CCI-MOC/haas,kylehogan/haas,SahilTikale/haas,kylehogan/hil,henn/hil