Add note about reading particle data to readme

README.md

@@ -62,3 +62,13 @@ print(ds)
 # Indexes: (9)
 # Attributes: (22)
 ```
+
+### Reading particle data
+
+By default, particle data isn't kept. Pass `keep_particles=True` as a
+keyword argument to `open_dataset` (for single files) or
+`open_mfdataset` (for multiple files):
+
+```python
+df = xr.open_dataset("0010.sdf", keep_particles=True)
+```