Document how to convert the project to use TypeScript

[fregante]

There have been a few issues regarding TypeScript, I think it makes sense to either add a conversion script to properly test it or at least add more information about what that entails.

browser-extension-template/readme.md

Line 78 in 569705f

- Adding TypeScript support is as easy as adding a `tsconfig.json` file and renaming your `.js` files to `.ts`; [sindresorhus/tsconfig][link-tsconfig] is also advised.