Update README.md

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

numpy_fft_1/README.md

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+# Numpy -- rfft and spectral power
+## Goal
+We want to calculate a well behaved power spectral density from a 1 dimensional time series. 
+
+Questions to [David Rotermund](mailto:davrot@uni-bremen.de)
+
+## Generation of test data 
+We will generate a sine wave with 10 Hz.
+
+```python
+import numpy as np
+
+time_series_length: int = 1000
+dt: float = 1.0 / 1000.0  # time resolution is 1ms
+sampling_frequency: float = 1.0 / dt
+frequency_hz: float = 10.0
+t: np.ndarray = np.arange(0, time_series_length) * dt
+y: np.ndarray = np.sin(t * 2 * np.pi * frequency_hz)
+```
+ 
+