Add Zenodo DOI reference badge to README

Author: jmert

README.md

@@ -1,8 +1,8 @@
 # UnixMmap
 
-| **Documentation**                                                         | **Build Status**                                             |
-|:-------------------------------------------------------------------------:|:------------------------------------------------------------:|
-| [![][docs-stable-img]][docs-stable-url] [![][docs-dev-img]][docs-dev-url] | [![][ci-img]][ci-url]                                        |
+| **Documentation**                                                         | **Build Status**      | **DOI**                 |
+|:-------------------------------------------------------------------------:|:---------------------:|:-----------------------:|
+| [![][docs-stable-img]][docs-stable-url] [![][docs-dev-img]][docs-dev-url] | [![][ci-img]][ci-url] | [![][doi-img]][doi-url] |
 
 `UnixMmap` is an alternative to the Julia standard library's `Mmap`, with the purpose of
 exposing the Unix memory-mapping interface.
@@ -55,6 +55,9 @@ julia> UnixMmap.mmap(Matrix{Float64}, (128, 3))
 [ci-img]: https://github.com/jmert/UnixMmap.jl/workflows/CI/badge.svg
 [ci-url]: https://github.com/jmert/UnixMmap.jl/actions
 
+[doi-img]: https://zenodo.org/badge/295815969.svg
+[doi-url]: https://zenodo.org/badge/latestdoi/295815969
+
 [codecov-img]: https://codecov.io/gh/jmert/UnixMmap.jl/branch/master/graph/badge.svg
 [codecov-url]: https://codecov.io/gh/jmert/UnixMmap.jl