diff --git a/doc/custom_roles.md b/doc/custom_roles.md
new file mode 100644
index 0000000..0513288
--- /dev/null
+++ b/doc/custom_roles.md
@@ -0,0 +1,42 @@
+# Custom roles
+
+- Custom ansible roles can be stored in `roles/custom`
+- To include custom roles into, you can list them in `custom_roles`.
+
+## Example
+
+```
+roles/custom/
+├── role_a
+│   └── tasks
+│       └── main.yml
+├── role_b
+│   └── tasks
+│       └── main.yml
+└── role_c
+    └── tasks
+        └── main.yml
+```
+
+inventory.yml
+
+```yaml
+all:
+  vars:
+    custom_roles:
+      - role_a
+      - role c
+desktops:
+  hosts:
+    10.0.2.[1:128]:
+laptops:
+  hosts:
+    10.0.32.[1:50]:
+  vars:
+    custom_roles:
+      - role_a
+      - role_b
+```
+
+All hosts (e.g. 10.0.2.14) will get `role_a` and `role_c`,
+but hosts in group laptop (e.g. 10.0.32.14) will get `role_a` and `role_b`