Added example to readme (#21)

Author: egpbos

README.md

@@ -8,6 +8,41 @@
 
 [fftw](http://www.fftw.org/) bindings for the [xtensor](https://github.com/QuantStack/xtensor) C++ multi-dimensional array library.
 
+## Example
+
+Calculate the derivative of a field `a` in Fourier space, e.g. a sine shaped field:
+
+```c++
+#include <xtensor-fftw/basic.hpp>   // rfft, irfft
+#include <xtensor-fftw/helper.hpp>  // rfftscale 
+#include <xtensor/xarray.hpp>
+#include <xtensor/xbuilder.hpp>     // xt::arange
+#include <xtensor/xmath.hpp>        // xt::sin, cos
+#include <complex>
+#include <xtensor/xio.hpp>
+
+// generate a sinusoid field
+double dx = M_PI/100;
+xt::xarray<double> x = xt::arange(0., 2*M_PI, dx);
+xt::xarray<double> sin = xt::sin(x);
+
+// transform to Fourier space
+auto sin_fs = xt::fftw::rfft(sin);
+
+// multiply by i*k
+std::complex<double> i {0, 1};
+auto k = xt::fftw::rfftscale<double>(sin.shape()[0], dx);
+xt::xarray< std::complex<double> > sin_derivative_fs = xt::eval(i * k * sin_fs);
+
+// transform back to normal space
+auto sin_derivative = xt::fftw::irfft(sin_derivative_fs);
+
+std::cout << "x:              " << x << std::endl;
+std::cout << "sin:            " << sin << std::endl;
+std::cout << "cos:            " << xt::cos(x) << std::endl;
+std::cout << "sin_derivative: " << sin_derivative << std::endl;
+```
+
 ## Usage
 
 _xtensor-fftw_ is a header-only library.