mention np.errstate context manager in exercise 31.

100 Numpy exercises.md

@@ -288,6 +288,13 @@ Z = np.ones(1)/0
 _ = np.seterr(**defaults)
 ```
 
+An equivalent way, with a context manager:
+
+```python
+with np.errstate(divide='ignore'):
+    Z = np.ones(1) / 0
+```
+
 #### 32. Is the following expressions true? (★☆☆)