add CITATION file

[caijun]

The added CITATION file will generate incidence citation info in the CRAN page of incidence.

The citation info is as follows

To cite the incidence package in publications use:

  Kamvar ZN, Cai J, Pulliam JRC, Schumacher J, Jombart T. Epidemic curves made easy using
  the R package incidence [version 1; referees: awaiting peer review]. F1000Research 2019,
  8:139. URL https://doi.org/10.12688/f1000research.18002.1.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Epidemic curves made easy using the R package incidence [version 1; referees: awaiting peer review]},
    author = {Zhian N. Kamvar and Jun Cai and Juliet R.C. Pulliam and Jakob Schumacher and Thibaut Jombart},
    journal = {F1000Research},
    year = {2019},
    volume = {8},
    number = {139},
    url = {https://doi.org/10.12688/f1000research.18002.1)},
  }

To cite the specific version of incidence package in publications use:

  Thibaut Jombart, Zhian N. Kamvar, Rich FitzJohn, Jun Cai, Sangeeta Bhatia, Jakob
  Schumacher and Juliet R.C. Pulliam (2019). incidence: Compute, Handle, Plot and Model
  Incidence of Dated Events. R package version 1.5.4.99.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {incidence: Compute, Handle, Plot and Model Incidence of Dated Events},
    author = {Thibaut Jombart and Zhian N. Kamvar and Rich FitzJohn and Jun Cai and Sangeeta Bhatia and Jakob Schumacher and Juliet R.C. Pulliam},
    year = {2019},
    note = {R package version 1.5.4.99},
    url = {http://www.repidemicsconsortium.org/incidence/},
  }

[zkamvar]

Thank you for this, @caijun!