Document app concept in README

README.md

@@ -2,7 +2,19 @@
 
 Application to manage and resolve [PURL](https://en.wikipedia.org/wiki/Persistent_uniform_resource_locator) links.
 
-## PURL Support
+## Documentation
+
+Until the documentation becomes large enough, this README will be used to
+documentation features and concepts.
+
+### Concept
+
+The application handles PURL domains. A domain is a collection of PURLs which
+you can think of as shortened URLs. When opened in a browser (a GET request),
+PURLs forward the user to the target URL which is configured by the PURLs
+maintainer.
+
+### PURL Support
 
 This project tries it's best to implement PURL support as it can be understood
 from a couple of hours researching the topic on the web. It is possible that