Add BibTex for citing E3SM

Also add a DOI link at top.

README.md

@@ -9,6 +9,8 @@ the most challenging and demanding climate-change research problems and
 Department of Energy mission needs while efficiently using DOE Leadership
 Computing Facilities.  
 
+DOI: [10.11578/E3SM/dc.20180418.36](http://dx.doi.org/10.11578/E3SM/dc.20180418.36)
+
 Please visit the [project website](https://e3sm.org) for further details.
 
 Table of Contents 
@@ -47,9 +49,25 @@ process for submitting pull requests.
 
 Acknowledgement
 --------------------------------------------------------------------------------
-The Energy Exascale Earth System Model (E3SM) Project should be acknowledged in 
-publications as the origin of model or data used in the research using [these 
-guidelines](https://e3sm.org/resources/policies/acknowledge-e3sm/).
+The Energy Exascale Earth System Model (E3SM) Project should be acknowledged in
+publications as the origin of the model using
+[these guidelines](https://e3sm.org/resources/policies/acknowledge-e3sm/).
+
+In addition, the software should be cited.  For your convenience,
+the following BibTeX entry is provided.
+```TeX
+@misc{e3sm-model,
+	title = {{Energy Exascale Earth System Model (E3SM)}},
+	author = {{E3SM Project}},
+	abstractNote = {{E3SM} is a state-of-the-art fully coupled model of the {E}arth's 
+		climate including important biogeochemical and cryospheric processes.},
+	howpublished = {[Computer Software] \url{https://dx.doi.org/10.11578/E3SM/dc.20180418.36}},
+	url = {https://dx.doi.org/10.11578/E3SM/dc.20180418.36},
+	doi = {10.11578/E3SM/dc.20180418.36},
+	year = 2018,
+	month = apr,
+}
+```
 
 License
 --------------------------------------------------------------------------------