Add the microkanren paper to the README

[rlouf]

The microkanren paper has helped me understand the library and I think it would be helpful for newcomers if it were mentioned in the "References" section of the README.

[majidaldo]

The microkanren paper has helped me understand the library and I think it would be helpful for newcomers if it were mentioned in the "References" section of the README.

i'll add that microKanren: A Lucid Little Logic Language with a Simple Complete Search was more specifically written for understanding.