diff --git a/README.md b/README.md
index 0cdb894..ed89d38 100644
--- a/README.md
+++ b/README.md
@@ -34,6 +34,13 @@
 - `--gitbucket.home=[DATA_DIR]`
 - `--temp_dir=[TEMP_DIR]`
 
+`TEMP_DIR` is used as the [temporary directory for the jetty application context](https://www.eclipse.org/jetty/documentation/9.3.x/ref-temporary-directories.html).
+This is the directory into which the gitbucket.war file is unpacked, the source
+files are compiled, etc.  
+If given this parameter **must** match the path of an existing directory
+or the application will quit reporting an error; if not given the path used
+will be a `tmp` directory inside the gitbucket home.
+
 You can also deploy gitbucket.war to a servlet container which supports Servlet 3.0 (like Jetty, Tomcat, JBoss, etc)
 
 For more information about installation on Mac or Windows Server (with IIS), or configuration of Apache or Nginx and also integration with other tools or services such as Jenkins or Slack, see [Wiki](https://github.com/gitbucket/gitbucket/wiki).