Update README with tutorial on creating an override

[zachsa]

Creating a pull request for the @arcgis/core library to include exports that weren't picked up automatically, I can see it would be beneficial to be walked through this process to make it easier to include an override

(I didn't realize that a new package.json was added during the JSPM build - i.e. https://ga.jspm.io/npm:@arcgis/[email protected]/package.json)

[guybedford]

Yeah a "step by step" for adding an export would be useful!