Add Overlay documentation (#76)

* Add Overlay documentation

* fix md link

* Enhancement matching proposals

* end of line .

* Update README.md

Co-authored-by: david <[email protected]>

* Update README.md

Co-authored-by: david <[email protected]>

---------

Co-authored-by: david <[email protected]>

README.md

@@ -1,10 +1,29 @@
 # Dockerized repository
 
 ## Configuration
+
+### Templated configuration
+
 Configure variables in .env. See [.env.example](.env.example) for template.
 Reasonable defaults are provided.
+We provide detailed documentation for [variable-expansion](https://doc.linkurious.com/admin-manual/latest/configure/#variable-expansion)
+
 You will most likely need to set your private repository host, if images are not available locally.
 
+This mode has limitations and will be dropped in the future.
+
+### Configuration overlay
+
+As an alternative, you may put a partial configuration file in your data folder: `data/config/production-overlay.json`.
+This file will be overlaid on top of your current production.json file. If no `production.json` file is provided, defaults will be generated.
+
+Overlay file will take precedence on anything included in the main configuration file.
+Values present in the overlay are no longer modifiable via the UI. Overlay file values always takes precedence.
+
+As the final configuration is the result of merging two files (overlay & configuration), it is possible to create an inconsistent configuration that will not pass validation, thus preventing Linkurious Enterprise from starting.
+
+See [Configuring Linkurious Enterpise](https://doc.linkurious.com/admin-manual/latest/configure/)
+
 ## Usage
   - Run with docker-compose
     ```