Document the feature for custom config file placement

docs/src/content/docs/reference/conf.mdx

@@ -8,10 +8,20 @@ of powerful customization options. Configuration is specified in the form of a
 `.pls.yml` YAML file (which means you can just use JSON if you hate YAML so
 much).
 
-This is the schema of the file.
+## Placement
+
+<Pls /> uses a cascading system for multiple config files. You can have config
+files at the directory level, at the repository level and at the global level.
+
+The global config file can be placed in the home directory at `~/.pls.yml`. If
+you prefer a clean home directory, you can set the `PLS_CONFIG` environment
+variable to a point to a config file placed elsewhere on your computer, in which
+case <Pls /> will not look for one in the home directory.
 
 ## Schema
 
+This is the schema of the file.
+
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     ### icons