* remove global npm install of create-next-app
* add npx to create-next-app command in examples
* add bash to shell snippets
* add yarn create to next-app command in examples
* fix READMEs named with lowercase
* change READMEs to use UPPERCASE
* Immplement the initial singleton Router.
* Use the new SingletonRouter for HMR error handling.
* Use SingletonRouter inside the Link.
* Create an example app using the Router.
* Make the url parameter optional in Router.push and Router.replace
* Add a section about next/router in the README.