diff --git a/readme.txt b/readme.txt
index c14dd72..9c86da9 100644
--- a/readme.txt
+++ b/readme.txt
@@ -16,7 +16,7 @@ How to use the shortcode:
 
 [cpdoi code="..."]Title of the paper[/doi]
 
-Using the [crossref.org](https://www.crossref.org/) api, the plugin send the doi code to [Crossref Metadata Search](https://search.crossref.org/), get the information and publish a full citation in apa standard with the shortcode [cpdoiresolve ...].
+The plugin send the doi code to [Crossref Metadata Search](https://search.crossref.org/), get the information and publish a full citation in apa standard with the shortcode [cpdoiresolve ...].
 How to use the shortcode:
 
 [cpdoiresolve code="..."]
@@ -31,6 +31,10 @@ if the paper has an arXiv version
 
 if the paper has a free pdf version
 
+## External service
+
+***Citations tools*** uses the [CrossRef Metadata API](http://search.crossref.org/help/api) to retrieve complete publication information using the doi code as a search query. CrossRef is a part of the [doi system](https://dx.doi.org/).
+
 == Installation ==
 1.  Extract the citations-tools.zip file and upload its contents to the /wp-content/plugins/ directory. Alternately, you can install directly from the Plugin directory within your WordPress Install.
 2. Activate the plugin through the "Plugins" menu in WordPress.