Create README.md

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

PyBind11/direct/README.md

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+# The fast and furious way
+{:.no_toc}
+
+<nav markdown="1" class="toc-class">
+* TOC
+{:toc}
+</nav>
+
+## Top
+
+Let us assume that you know what you are doing. And also let us assume that you notices how extremely slow the "correct" way of communication between Python and C++ is. Well the following section is for you...
+
+Questions to [David Rotermund](mailto:davrot@uni-bremen.de)
+
+
+
+## On the Python side
+
+```python
+# If it is a torch tensor then make a "view" to its numpy core
+np_input: np.ndarray = input.contiguous().detach().numpy()
+
+# We need to make sure that the numpy ndarray is C_CONTIGUOUS. 
+# If not then use numpy.ascontiguousarray() to make it so
+assert np_input.flags["C_CONTIGUOUS"] is True
+
+# Input is a 4d ndarray. And I will make sure that this is really the case
+assert np_input.ndim == 4
+
+# Now I extract the pointer to the data memory of the ndarray 
+np_input_pointer, _ = np_input.__array_interface__["data"]
+
+# Also I need the shape information for the C++ program. 
+np_input_dim_0: int = np_input.shape[0]
+np_input_dim_1: int = np_input.shape[1]
+np_input_dim_2: int = np_input.shape[2]
+np_input_dim_3: int = np_input.shape[3]
+```
+
+## On the C++ side
+
+Your C++ method needs to accept these arguments
+
+```cpp
+int64_t np_input_pointer_addr, 
+int64_t np_input_dim_0,
+int64_t np_input_dim_1, 
+int64_t np_input_dim_2, 
+int64_t np_input_dim_3,
+```
+
+Inside your C++ method you convert the address into a pointer. **BE WARNED:** Make absolutely sure that the dtype of the np.ndarray is correctly reflected in the pointer type
+
+dtype=np.float32 --> float
+dtype=np.float64 --> double
+dtype=np.uint64 --> uint64
+
+If you fuck this up then this will end in tears!
+
+```cpp
+float *np_input_pointer = (float *)np_input_pointer_addr;
+
+// Input
+assert((np_input_pointer != nullptr));
+assert((np_input_dim_0 > 0));
+assert((np_input_dim_1 > 0));
+assert((np_input_dim_2 > 0));
+assert((np_input_dim_3 > 0));
+```
+
+Don't forget that C Contiguous is just a complicated way of saying Row-major order memory layout [Row- and column-major order](https://en.wikipedia.org/wiki/Row-_and_column-major_order).
+
+$$M[a,b,c,d] = M[\eta_a \cdot a + \eta_b \cdot  b + \eta_c \cdot  c + d]$$
+
+with
+
+$$\eta_c = n_d$$
+
+$$\eta_b = \eta_c \cdot n_c$$
+
+$$\eta_a = \eta_b \cdot n_b$$