docs: add Debian package badge

stream-zip is now in sid/unstable, discussed slightly in
#136

README.md

@@ -1,7 +1,7 @@
 <!-- --8<-- [start:intro] -->
 # stream-zip
 
-[![conda-forge package](https://img.shields.io/conda/v/conda-forge/stream-zip?label=conda-forge&color=%234c1)](https://anaconda.org/conda-forge/stream-zip) [![PyPI package](https://img.shields.io/pypi/v/stream-zip?label=PyPI%20package&color=%234c1)](https://pypi.org/project/stream-zip/) [![Test suite](https://img.shields.io/github/actions/workflow/status/uktrade/stream-zip/test.yml?label=Test%20suite)](https://github.com/uktrade/stream-zip/actions/workflows/test.yml) [![Code coverage](https://img.shields.io/codecov/c/github/uktrade/stream-zip?label=Code%20coverage)](https://app.codecov.io/gh/uktrade/stream-zip)
+[![Debian (unstable) package](https://img.shields.io/debian/v/python3-stream-zip/sid?label=Debian%20%28unstable%29)](https://packages.debian.org/sid/python3-stream-zip) [![conda-forge package](https://img.shields.io/conda/v/conda-forge/stream-zip?label=conda-forge&color=%234c1)](https://anaconda.org/conda-forge/stream-zip) [![PyPI package](https://img.shields.io/pypi/v/stream-zip?label=PyPI%20package&color=%234c1)](https://pypi.org/project/stream-zip/) [![Test suite](https://img.shields.io/github/actions/workflow/status/uktrade/stream-zip/test.yml?label=Test%20suite)](https://github.com/uktrade/stream-zip/actions/workflows/test.yml) [![Code coverage](https://img.shields.io/codecov/c/github/uktrade/stream-zip?label=Code%20coverage)](https://app.codecov.io/gh/uktrade/stream-zip)
 
 Python function to construct a ZIP archive on the fly - without having to store the entire ZIP in memory or disk. This is useful in memory-constrained environments, or when you would like to start returning compressed data before you've even retrieved all the uncompressed data. Generating ZIPs on-demand in a web server is a typical use case for stream-zip.