Add Zenodo DOI badge and DOI to citation file

Author: codeplea

CITATION.cff

@@ -5,6 +5,7 @@ authors:
   - family-names: "Van Winkle"
     given-names: "Lewis"
 license: Zlib
+doi: 10.5281/zenodo.21817114
 repository-code: "https://github.com/codeplea/tinyexpr"
 url: "https://codeplea.com/tinyexpr"
 keywords:

README.md

@@ -3,6 +3,7 @@
 # TinyExpr
 
 [![CI](https://github.com/codeplea/tinyexpr/actions/workflows/ci.yml/badge.svg?branch=master)](https://github.com/codeplea/tinyexpr/actions/workflows/ci.yml)
+[![DOI](https://zenodo.org/badge/50113629.svg)](https://doi.org/10.5281/zenodo.21817114)
 
 TinyExpr is a very small recursive descent parser and evaluation engine for
 math expressions. It's handy when you want to add the ability to evaluate