Started documenting input section of configuration

src/site/markdown/configuration.md.vm

@@ -37,11 +37,28 @@ $h2 Configuration Sections
 
 $h3 Input
 
+The input section defines the input data for which random numbers will be generated.
+
 ```
 input:
   name: data/input.csv
 ```
 
+The input file is a simple csv file in which the identifiers are defined. The only column in this file is a list of
+identifiers.
+
+Example:
+
+```
+A001
+A002
+B003
+```
+
+This input file defines 3 identifiers.
+
+**Important** The file may not contain any duplicated identifiers.
+
 $h3 Output
 
 ```