diff --git a/README.md b/README.md
index 088613d..76594c4 100644
--- a/README.md
+++ b/README.md
@@ -37,6 +37,9 @@ Next, add a [shell alias](https://shapeshed.com/unix-alias/):
 alias mydocker='docker build -t mydocker . && docker run --cap-add="SYS_ADMIN" mydocker'
 ```
 
+(The `--cap-add="SYS_ADMIN"` flag is required to create
+[PID Namespaces](https://man7.org/linux/man-pages/man7/pid_namespaces.7.html))
+
 You can now execute your program like this:
 
 ```sh