Add CITATION file. Closes #229

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>

inst/CITATION

@@ -0,0 +1,19 @@
+citHeader(paste0("See AUTHORS file for a list of all authors.\n",
+                 "To cite git2r in publications, please use:"))
+
+year <- "2016"
+vers <- "0.14.0"
+pkg.url <- "https://CRAN.R-project.org/package=git2r"
+
+citEntry(entry = "Manual",
+  author       = "Stefan Widgren and others",
+  title        = "{git2r}: Provides Access to Git Repositories",
+  year         = year,
+  note         = paste0("R package version ", vers),
+  url          = pkg.url,
+  textVersion  =
+  paste(paste0("Widgren, S., et al. (", year, ")"),
+        "git2r: Provides Access to Git Repositories.",
+        paste0("R package version ", vers, "."),
+        paste0("URL ", pkg.url, "."))
+)