ENH: Add DOI and citation info

CITATION.cff

@@ -0,0 +1,11 @@
+cff-version: 1.2.0
+message: "If you use this software, please cite it as below."
+authors:
+- family-names: "Gilbert"
+  given-names: "Matthew"
+  orcid: "https://orcid.org/0000-0002-5094-6940"
+title: "blp: Pythonic interface for Bloomberg Open API"
+version: 0.0.1
+doi: 10.5281/zenodo.5187880
+date-released: 2021-08-12
+url: "https://github.com/matthewgilbert/blp"

README.md

@@ -2,6 +2,7 @@
 [![Conda](https://img.shields.io/conda/v/conda-forge/blp)](https://anaconda.org/conda-forge/blp)
 [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0)
 [![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/psf/black)
+[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.5187880.svg)](https://doi.org/10.5281/zenodo.5187880)
 
 # About