docs(examples): added file upload example

[marvinwu]

upload file and images

This is a simple example of using the remix buildin uploadHandler and Form with multipart data to upload an image file and display it,
it also show a simple(though not efficient way) of integrate with cloudinary without writing custom handler.

the relevent files are:

├── app
│   ├── routes
│   │   ├── cloudinary-upload.tsx // upload to cloudinary
│   │   └── local-upload.tsx // local upload using build in [createfileuploadhandler](https://remix.run/docs/en/v1/api/remix#unstable_createfileuploadhandler)
│   └── utils
│       └── utils.server.ts  // init cloudinary nodejs client on server side
|── .env // holds cloudinary credentails

steps to set up cloudinary

• sign up a free cloudinary account
• get the cloudname, api key and api secret from dashboard
• copy the .env.sample to .env and populate the credentials

[remix-cla-bot]

Hi @marvinwu,

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[remix-cla-bot]

Thank you for signing the Contributor License Agreement. Let's get this merged! 🥳

[ryanflorence]

@kentcdodds can you review this example and merge when you're ready?

[kentcdodds]

Looking good! I've got a few bits of feedback for this one.

[kentcdodds]

Sweet! This is very cool 👍

[kentcdodds]

Thanks everyone 👏👏

[silvenon]

Love this example! Just a question about environment variables: according to Remix docs dotenv should only be used when starting the development script via node -r, see server environment variables. However, I prefer your way much better. Is there something wrong with it then?

[jacob-ebey]

I always avoid calling it via code. It's a potential source of bugs if you ship a ".env" to production by accident.

[kentcdodds]

Agreed! Fixed 👍