diff --git a/README.md b/README.md
index 54fe578..87b3396 100644
--- a/README.md
+++ b/README.md
@@ -10,4 +10,9 @@
 
 Remove attachment(s) when delete the item(s) or collection.
 
-Download xpi from [https://github.com/redleafnew/delitemwithatt/releases](https://github.com/redleafnew/delitemwithatt/releases), and click Tools-Addons-Install Add-on From File in Zotero or JurisM to install the extension. After the installation, right click the item(s) `Delete Item(s) with Attachment(s)` shows, when confirm, the item(s) with attachment(s) will be deleted; right click the collection `Delete Collections with Attachments` shows, when confirm, the collection and the item(s) with attachment(s) in the selected collection will be deleted.
\ No newline at end of file
+Download xpi from [https://github.com/redleafnew/delitemwithatt/releases](https://github.com/redleafnew/delitemwithatt/releases), and click Tools-Addons-Install Add-on From File in Zotero or JurisM to install the extension. After the installation, right click the item(s) `Delete Item(s) with Attachment(s)` shows, when confirm, the item(s) with attachment(s) will be deleted; right click the collection `Delete Collections with Attachments` shows, when confirm, the collection and the item(s) with attachment(s) in the selected collection will be deleted.
+
+# License
+The source code is released under GNU General Public License, version 3.0
+
+Contributions preferably through pull requests are welcome!