Add citation info and crude README

CITATION.cff

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+cff-version: 1.2.0
+message: "If you use this software, please cite it using the metadata supplied in the CITATION.cff file."
+doi: 10.5281/zenodo.4279483
+title: "OpenMDAO-NSGA"
+authors:
+  - family-names: Vidner
+    given-names: Olle
+    affiliation: Linköping University
+    orcid: https://orcid.org/0000-0002-1157-2480
+license: MIT
+repository-code: https://github.com/ovidner/openmdao-nsga

README.md

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-# openmdao-nsga
-NSGA-II and NSGA-III implementations for OpenMDAO
+# OpenMDAO-NSGA
+
+[![DOI](https://zenodo.org/badge/DOI/10/f762.svg)](https://doi.org/f762)
+
+NSGA-II and NSGA-III implementations for OpenMDAO.
+
+Some notable features include:
+
+* Support for continuous, integer, discrete and categorical variables.
+* Support for Pareto-optimization of multi-objective problems.
+* Constraint handling without parameters (using the method described in the original NSGA-II paper).
+* Flexible termination criteria.
+
+Do note that the performance for discrete problems will probably be bad, but if you are looking for a fairly robust *shotgun approach* for a variety of different problems, OpenMDAO-NSGA might serve you well enough.
+
+## Installation
+
+This assumes you are using Conda. Run this in your environment to install:
+
+    conda install ovidner::openmdao-nsga
+
+## Usage example
+
+```python
+import openmdao.api as om
+import omnsga
+
+prob = om.Problem()
+
+...
+
+# Real or integer design var
+prob.model.add_design_var("x_1", lower=0, upper=1)
+# Discrete design var
+omnsga.add_design_var(prob.model, "x_2", values={0, 1, 4}, type=omnsga.VariableType.ORDINAL)
+# Categorical design var
+omnsga.add_design_var(prob.model, "x_3", values={True, False, None}, type=omnsga.VariableType.NOMINAL)
+
+prob.model.add_constraint("g", lower=0, upper=1)
+# Maximize f_1
+prob.model.add_objective("f_1", scaler=-1)
+# Minimize f_2
+prob.model.add_objective("f_2")
+
+prob.driver = omnsga.Nsga2Driver(
+    termination_criterion=omnsga.MaxEvaluationsCriterion(500),
+)
+
+prob.run_driver()
+```