DOC: added citation guide

Added a citation guide for apexpy.

docs/citing.rst

@@ -0,0 +1,29 @@
+Citations
+=========
+
+When referring to this software package, please be sure to include the package
+and specify which version you used `<https://doi.org/10.5281/zenodo.1214206>`_.
+Note that this DOI will always point to the latest version of the code.  A list
+of DOIs for all versions can be found at the Zenodo page above. The version can
+by found by printing :py:attr:`apexpy.__version__`.  We also recommend citing a
+reference to the coordinate system you use.  More relevant references can be
+found in :ref:`authors`.
+
+Example for citation in BibTex for a generalized version:
+
+.. code::
+
+  @misc{apexpy,
+    author       = {van der Meeren, Christer and
+                    Laundal, Karl M. and
+		    Burrell, Angeline G. and
+		    Lamarche, Leslie and
+		    Starr, Gregory and
+		    Reimer, Ashton and
+		    Morschhauser, Achim and
+		    Michaelis, Ingo},
+    title        = {ApexPy vX.Y.Z},
+    year         = 2024,
+    doi          = {10.5281/zenodo.1214206},
+    url          = {https://github.com/aburrell/apexpy}
+  }