Add CITATION file

[dwhswenson]

Is this the current preferred citation for LOMAP? https://link.springer.com/article/10.1007/s10822-013-9678-y

(This repo involved rewriting the original OpenEye/Schrödinger based implementation using RDkit; not sure if there's a newer citation for that, or what the preferred citations are.)

[IAlibay]

pinging @davidlmobley - what would be the preferred citation(s) here?

[davidlmobley]

Ah, yes, that's the original one and there has never been an updated pub, so that's the preferred citation.