Update README.md

Signed-off-by: David Rotermund <54365609+davrot@users.noreply.github.com>

numpy/where/README.md

@@ -25,3 +25,92 @@ numpy.where(condition, [x, y, ]/)
 > **x**, **y** : array_like
 >  Values from which to choose. x, y and condition need to be broadcastable to some shape.
 
+## Finding indices 
+
+### 2d example
+
+```python
+import numpy as np
+
+a = np.arange(0, 15).reshape((5, 3))
+
+print(a)
+print()
+
+w = np.where(a > 7)
+print(w)
+```
+
+Output:
+
+```python
+[[ 0  1  2]
+ [ 3  4  5]
+ [ 6  7  8]
+ [ 9 10 11]
+ [12 13 14]]
+
+(array([2, 3, 3, 3, 4, 4, 4]), array([2, 0, 1, 2, 0, 1, 2]))
+```
+
+### 3d example
+
+```python
+import numpy as np
+
+a = np.arange(0, 30).reshape((5, 3, 2))
+
+w = np.where(a > 15)
+print(w)
+```
+
+Output:
+
+```python
+(array([2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4]), array([2, 2, 0, 0, 1, 1, 2, 2, 0, 0, 1, 1, 2, 2]), array([0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1]))
+```
+
+Using the found indices: 
+
+```python
+import numpy as np
+
+a = np.arange(0, 30).reshape((5, 3, 2))
+
+idx = np.where(a > 15)
+a[idx] = 42
+print(a)
+```
+
+Output: 
+
+```python
+[[[ 0  1]
+  [ 2  3]
+  [ 4  5]]
+
+ [[ 6  7]
+  [ 8  9]
+  [10 11]]
+
+ [[12 13]
+  [14 15]
+  [42 42]]
+
+ [[42 42]
+  [42 42]
+  [42 42]]
+
+ [[42 42]
+  [42 42]
+  [42 42]]]
+```
+
+```python
+import numpy as np
+
+a = np.arange(0, 30).reshape((5, 3, 2))
+
+a[np.where(a > 15)] = 42
+print(a)
+```