Add ENV sample file

README.md has a section about setting up ENV

> **ENVs**
> To populate project with environment variables create files .env.development for local development and .env.production for final build.

To bootstrap a development environment, I suggest adding a `.env.sample` and a brief description of each variable - the case of `HJ_*` for Hotjar.

With this implemented, developers can create `env` files by running:

```
$ cp .env.sample .env.development
```

then update values.