Create README.md

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

numpy/advanced_indexing/README.md

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+# Advanced Indexing
+{:.no_toc}
+
+<nav markdown="1" class="toc-class">
+* TOC
+{:toc}
+</nav>
+
+## The goal
+
+Beside slicing there is something called advanced indexing 
+
+Questions to [David Rotermund](mailto:davrot@uni-bremen.de)
+
+## Boolean Array​
+
+We can use Boolean arrays for more complicate indexing: 
+
+```python
+import numpy as np
+
+a = np.arange(1,10).reshape(3,3)
+b = np.zeros_like(a)
+
+b[a.sum(axis=1) > 6, :] = 1
+
+print(a)
+print()
+print(b)
+```
+
+Output:
+
+```python
+[[1 2 3]
+ [4 5 6]
+ [7 8 9]]
+
+[[0 0 0]
+ [1 1 1]
+ [1 1 1]]
+```
+
+Behind the curtains more or less this happens:​
+
+```python
+import numpy as np
+
+a = np.arange(1, 10).reshape(3, 3)
+b = np.zeros_like(a)
+
+temp_0 = a.sum(axis=1)
+temp_1 = temp_0 > 6
+temp_2 = np.nonzero(temp_1)
+b[temp_2] = 1
+
+print(temp_0)
+print()
+print(temp_1)
+print()
+print(temp_2)
+print()
+print(b)
+```
+
+Output:
+
+```python
+[ 6 15 24]
+
+[False  True  True]
+
+(array([1, 2]),)
+
+[[0 0 0]
+ [1 1 1]
+ [1 1 1]]
+```
+