Add an example

[isuruf]

https://github.com/symengine/symengine.py/blob/62a0d89b5b9ad00814c5cd2f72e697aa1c21dccd/symengine/tests/test_lambdify.py#L818-L824

Here's an example where you can use scipy's integrate with symengine providing a fast native compiled function instead of a python function. (Maybe we can compare a python function and symengine function)

[isuruf]

cc @HaoZeke