docs(charts): add documentation for hooks.
docs(charts): add documentation for hooks.

Closes #938

technosophos/k8s-helm,rbwsam/helm,kdada/helm,technosophos/deployment-manager,fibonacci1729/helm,vaikas-google/deployment-manager,qwangrepos/helm,adamreese/deployment-manager,flynnduism/helm,FengyunPan/helm,technosophos/deployment-manager,yan234280533/helm,technosophos/k8s-helm,qwangrepos/helm,appscode/helm,bradj/helm,adamreese/deployment-manager,thomastaylor312/helm,flynnduism/helm,adamreese/deployment-manager,bacongobbler/helm,bacongobbler/helm,eicnix/helm,jascott1/helm,SlickNik/helm,bradj/helm,sushilkm/helm,iamzhout/helm,FengyunPan/helm,technosophos/deployment-manager,eicnix/helm,iamzhout/helm,michelleN/deployment-manager,helm/helm,adamreese/helm,adamreese/helm,fibonacci1729/helm,vaikas-google/deployment-manager,helm/helm,adamreese/deployment-manager,sushilkm/helm,kdada/helm,michelleN/deployment-manager,rbwsam/helm,michelleN/deployment-manager,adamreese/deployment-manager,michelleN/deployment-manager,thomastaylor312/helm,technosophos/deployment-manager,yan234280533/helm,SlickNik/helm,appscode/helm,technosophos/deployment-manager,jascott1/helm