doc: add installation method documentation

docs/running_psalm/installation.md

@@ -28,7 +28,7 @@ While Psalm can figure out the types used by various libraries based on
 their source code and docblocks, it works even better with custom-tailored types
 provided by Psalm plugins.
 
-Check out the [list of existing plugins on Packagist](https://packagist.org/?type=psalm-plugin). 
+Check out the [list of existing plugins on Packagist](https://packagist.org/?type=psalm-plugin).
 Install them with `composer require --dev <plugin/package> && vendor/bin/psalm-plugin enable <plugin/package>`
 
 Read more about plugins in [Using Plugins chapter](plugins/using_plugins.md).
@@ -51,3 +51,54 @@ Alternatively, you can use Composer to install the Phar:
 ```bash
 composer require --dev psalm/phar
 ```
+
+## Using a Snapshot Build
+
+To obtain the latest development version of Psalm, a snapshot build can be
+downloaded from GitHub at https://github.com/vimeo/psalm/archive/refs/heads/5.x.zip
+
+Snapshot builds typically include only the source code and exclude the `.git`
+directory. As a result, Psalm cannot automatically determine its version or
+revision when executing `composer install`, since this information is usually
+derived from the `git` repository metadata that Composer relies on.
+
+To successfully install Psalm from a snapshot build, it is necessary to
+manually specify the version using the `COMPOSER_ROOT_VERSION` environment
+variable. This allows Composer to proceed with the installation of Psalm.
+
+The command to do so is as follows:
+
+```bash
+COMPOSER_ROOT_VERSION=5-dev composer install
+```
+
+## Installing Psalm on Linux
+
+Psalm is available across various Linux distributions, each typically equipped
+with its own package management system. To install Psalm, refer to the
+documentation specific to your distribution's package manager.
+
+### Installation on Nix
+
+For users of Nix, Psalm is readily [available](https://search.nixos.org/packages?channel=23.11&from=0&size=50&sort=relevance&type=packages&query=psalm)
+and can be installed or run using several methods.
+
+To run Psalm directly, use the following command:
+
+```bash
+nix run github:NixOS/nixpkgs-unstable#php.packages.psalm
+```
+
+For an ephemeral shell environment with Psalm, either of the following commands
+can be used:
+
+```bash
+nix-shell -p php.packages.psalm
+```
+
+```bash
+nix shell github:NixOS/nixpkgs-unstable#php.packages.psalm
+```
+
+Psalm builds on Nix are reproducible, ensuring that executing the same command
+on any system yields an identical build of Psalm.