docs: expand t3-app README (t3-oss#248)

template/base/README.md

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 # Create T3 App
 
 This is an app bootstrapped according to the [init.tips](https://init.tips) stack, also known as the T3-Stack.
+
+## Why are there `.js` files in here?
+
+As per [T3-Axiom #3](https://github.com/t3-oss/create-t3-app/tree/next#3-typesafety-isnt-optional), we believe take typesafety as a first class citizen. Unfortunately, not all frameworks and plugins support TypeScript which means some of the configuration files have to be `.js` files.
+
+We try to emphasize that these files are javascript for a reason, by explicitly declaring its type (`cjs` or `mjs`) depending on what's supported by the library it is used by. Also, all the `js` files in this project are still typechecked using a `@ts-check` comment at the top.
+
+## What's next? How do I make an app with this?
+
+We try to keep this project as simple as possible, so you can start with the most basic configuration and then move on to more advanced configuration.
+
+If you are not familiar with the different technologies used in this project, please refer to the respective docs. If you still are in the wind, please join our [Discord](https://t3.gg/discord) and ask for help.
+
+- [Next-Auth.js](https://next-auth.js.org)
+- [Prisma](https://prisma.io)
+- [TailwindCSS](https://tailwindcss.com)
+- [tRPC](https://trpc.io) (using @next version? [see v10 docs here](https://alpha.trpc.io))
+
+## Useful resources
+
+Here are some resources that we commonly refer to:
+
+- [Protecting routes with Next-Auth.js](https://next-auth.js.org/configuration/nextjs#unstable_getserversession)