docs: add extras installation instructions

Author: jokasimr

docs/user-guide/installation.md

@@ -1,6 +1,6 @@
 # Installation
 
-To install ESSreflectometry and all of its dependencies, use
+To install ESSreflectometry and all of its required dependencies, use
 
 `````{tab-set}
 ````{tab-item} pip
@@ -14,3 +14,19 @@ conda install -c conda-forge -c scipp essreflectometry
 ```
 ````
 `````
+
+To install ESSreflectometry and all of its dependencies,
+including dependencies used in the graphical user interface for batch reduction, use
+
+`````{tab-set}
+````{tab-item} pip
+```sh
+pip install essreflectometry[all]
+```
+````
+````{tab-item} conda
+```sh
+conda install -c conda-forge -c scipp essreflectometry[all]
+```
+````
+`````