README: add "best practices" for callback section
README: add "best practices" for callback section

Closes #4.

EvolveLabs/electron-weak,robcolburn/node-weak,robcolburn/node-weak,TooTallNate/node-weak,KenanSulayman/node-weak,kkoopa/node-weak,fastest963/node-weak,EvolveLabs/electron-weak,unbornchikken/node-weak,fastest963/node-weak,robcolburn/node-weak,KenanSulayman/node-weak,unbornchikken/node-weak,kkoopa/node-weak,TooTallNate/node-weak,unbornchikken/node-weak,EvolveLabs/electron-weak,KenanSulayman/node-weak,fastest963/node-weak,TooTallNate/node-weak,kkoopa/node-weak