add description of an optional config file

There has been the request for an example how to set up and use
a configuration file.[1]  The documentation now includes both a
minimal example, as well as points to an example in a larger
project.

[1] fortran-lang#142

Signed-off-by: Norwid Behrnd <nbehrnd@yahoo.com>

README.md

@@ -113,6 +113,21 @@ For more options, read
 fprettify -h
 ```
 
+## Configuration file
+
+Adjusted parameters of `fprettify` can be stored in a configuration file in a
+pattern like
+
+```sh
+# Explicitly define / overwrite the program's defaults e.g.
+indent=2
+whitespace=0
+```
+
+The rules are activated by the optional parameter `-c` and subsequent indication
+of the config file in question. For one example of application, see the setup of
+[ABIN](https://github.com/PHOTOX/ABIN/blob/master/.fprettify.rc).
+
 ## Editor integration
 
 For editor integration, use