[meta] Add citation instructions

[ejgallego]

There is a (not yet) published paper for coq-lsp, upload to ArXiV and allow people to cite it. A preliminary biber entry is:

@unpublished{,
  author = {Gallego Arias, Emilio Jesús},
  title  = {Flèche: Incremental Validation for Hybrid Formal Documents},
  year = {2024}
}

Moreover, provide a biber software entry for it too.

@Software{rocq:rocq-lsp,
  author   = {Gallego Arias, Emilio Jesús and Caglayan, Ali, and Itzhaky, Shachar, and Blanqui, Fréderic and Lepigre, Rodolphe and others},
  title    = {rocq-lsp: a language server for the Rocq Prover},
  url      = {https://github.com/ejgallego/coq-lsp},
  urldate  = {2025-04-01},
  year     = {2025},
}

Thanks to "Alexandre J" on Zulip for reminding us of this.