diff --git a/README.md b/README.md
index d57d411..5dedbb7 100644
--- a/README.md
+++ b/README.md
@@ -30,6 +30,8 @@ To start parsing command-line arguments, create an ```ArgumentParser```.
 argparse::ArgumentParser program("program name");
 ```
 
+**NOTE:** There is an optional second argument to the `ArgumentParser` which is the program version. Example: `argparse::ArgumentParser program("libfoo", "1.9.0");`
+
 To add a new argument, simply call ```.add_argument(...)```. You can provide a variadic list of argument names that you want to group together, e.g., ```-v``` and ```--verbose```
 
 ```cpp