clarify CICE citation info in doc (CICE-Consortium#487)

doc/source/intro/citing.rst

@@ -5,14 +5,26 @@
 Citing the CICE code
 ====================
 
-If you use the CICE code, please cite the version you are using with the CICE 
-Digital Object Identifier (DOI):
+Each individual release has its own Digital Object Identifier (DOI),
+e.g. CICE v6.1.2 has DOI 10.5281/zenodo.3888653. All versions of
+this lineage (e.g. CICE6) can be cited by using the DOI
+10.5281/zenodo.1205674 (https://zenodo.org/record/1205674). This DOI
+represents all v6 releases, and will always resolve to the latest one.
+More information can be found by following the DOI link to zenodo.
 
-DOI:10.5281/zenodo.1205674 (https://zenodo.org/record/1205674) 
+If you use CICE, please cite the version number of the code you
+are using or modifying.
 
-This DOI can be used to cite all CICE versions and the URL will default to the most recent version. 
-However, each released version of CICE will also receive its own, unique DOI that can be 
-used for citations as well.
+If using code from the CICE-Consortium repository ``master`` branch
+that includes modifications
+that have not yet been released with a version number, then in
+addition to the most recent version number, the hash at time of
+download can be cited, determined by executing the command ``git log``
+in your clone.
 
-Please also make the CICE Consortium aware of any publications and model use.
+A hash can also be cited for your own modifications, once they have
+been committed to a repository branch.
+
+Please also make the CICE Consortium aware of any publications and
+model use.