docs: Add Nix and Flakes (#123)

<!--
Fill in the relevant information below to help triage your issue.

Assuming the current release is 1.5.0, the next patch release is 1.5.1,
the next minor is 1.6.0 and the next major is 2.0.0, The Current release
branch will be `1.5.x`, the next minor branch will be `1.6.x`, and the
next major branch will be `2.0.x`

Pick the target branch based on the following criteria:
  * Documentation improvement: Current release branch 1.5.x
  * Bugfix: Current release branch 1.5.x
* QA improvement (additional tests, CS fixes, etc.) that does not change
code
    behavior: Next minor 1.6.x
  * New feature, or refactor of existing code: Next minor 1.6.x
  * Backwards incompatible features and refactoring: Next major 2.0.x

You MUST provide a signoff in your commits for us to be able to accept
your
patch; you can do this by providing either the --signoff or -s flag when
using
"git commit". Please see the project contributing guide and
https://developercertificate.org for details.
-->

|    Q          |   A
|-------------- | ------
| Documentation | yes
| Bugfix        | yes/no
| BC Break      | yes/no
| New Feature   | yes/no
| RFC           | yes/no
| QA            | yes/no

### Description

<!--

Tell us about why this change is necessary:
- Are you fixing a bug or providing a failing unit test to demonstrate a
bug?
  - How do you reproduce it?
  - What did you expect to happen?
  - What actually happened?
  - TARGET THE CURRENT RELEASE BRANCH

- Are you adding documentation?
  - TARGET THE CURRENT RELEASE BRANCH

- Are you providing a QA improvement (additional tests, CS fixes, etc.)
that
  does not change behavior?
  - Explain why the changes are necessary
  - TARGET THE NEXT MINOR BRANCH

- Are you fixing a BC Break?
  - How do you reproduce it?
  - What was the previous behavior?
  - What is the current behavior?
  - TARGET THE CURRENT RELEASE BRANCH

- Are you adding something the library currently does not support?
  - Why should it be added?
  - What will it enable?
  - How will the code be used?
  - TARGET THE NEXT MINOR BRANCH OR THE NEXT MAJOR IF BC WILL BE BROKEN

- Are you refactoring code?
  - Why do you feel the refactor is necessary?
  - What types of refactoring are you doing?
  - TARGET THE NEXT MINOR BRANCH OR THE NEXT MAJOR IF BC WILL BE BROKEN
-->


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit


- **Documentation**
- Introduced a dedicated section for integrating the Nix package manager
into the project workflow.
- Provided clear instructions for starting the development environment
with Nix and using `direnv` for automatic setup.
- Added a guide for installing Nix on Linux and macOS, highlighting its
advantages over traditional package managers.
- Enhanced documentation with links to external resources for more
information on Nix.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Miklós Galicz <[email protected]>
Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

README.md

@@ -123,6 +123,32 @@ The database is populated on first start, and if you want to have images availab
 
 This will go through the contents of the database and will use placeholder images to "generate" the necessary images to the designated S3 compatible file hosting solution designated in the `.env` file.
 
+## With Nix
+
+This project has a Nix `flake.nix` with the full development environment configuration in it. Start it with:
+
+```sh
+nix develop
+```
+
+If you want to start the development environment automatically when entering the directory, [install direnv](https://direnv.net/docs/installation.html) and run `direnv allow` in this directory.
+
+
+### First time installing Nix
+
+With Nix installed, you do not need to install Go, ASDF, node, npm, bun or other development tools. **Nix package manager** will handle it for you, and makes sure the versions are correct for this project. This has been tested on both Linux and macOS.
+Nix is a package manager for the whole system/development environment, not just 1 part of it, like NPM is for Node. There is also NixOS, which works in the same way but for the whole OS, making it declarative and versioned. The config in this project works for both. With the Nix Flake in this project, you are using Nix only to manage the development environment.
+
+### Install Nix package manager
+
+For both Linux and macOS, you can use the installer from determinate systems. On the site [Zero to nix](https://zero-to-nix.com/start) you can find more info to get started.
+
+```sh
+curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix | sh -s -- install
+```
+
+If you installed Nix differently, make sure that you **[enable flakes](https://nixos.wiki/wiki/Flakes)**. [Flake concepts explained](https://zero-to-nix.com/concepts/flakes). Flakes are experimental, but highly recommended and included with Nix for years.
+
 ## License
 
 This project is licensed under the MIT License - see the [LICENSE.md](LICENSE.md) file for details