Contour integral over unit circle.

Question

I'm completely new to complex analysis. I got this contour integral when I am reading a paper. The integral is given as follows, $$lim_{ r \downarrow 1} \oint_{\left|\xi_{1}\right|=1} \frac{\left(1+h \xi_{1}\right)^l\left(\xi_{1}+h\right)^l}{\xi^l_{1}\left(\xi_{1}-r \xi_{2}\right)^{2}} \mathrm{~d} \xi_{1} = \sum_{i_{1}+i_{2}+i_{3}=l - 1} \frac{\left(i_{3}+1\right) ! h^{l+i_{1}-i_{2}}}{\left(l-i_{1}\right) !\left(l-i_{2}\right) !}$$ where $l \ge 1$, $\xi_2$ and $h$ are constants. Any tips or explanation will be really appreciated. Thanks in advance!