Removes requirement for a post-build.js file in the root of the project for a custom post build step in gatsby build. Instead it requires for a postBuild export in the gatsby-node file in the root of the domain, where the webpack custom configuration is also stored.

Added a better error message for custom post-build actions defined in gatsby-node.js, explains to user that file cannot be used.

Updated README.md by removing post-build.js option in the project structure and including a new section explaining how to include a post-build function for users.

Correct some errors in the README.

Changed strings from double to single quotes to avoid eslint error.