readme: add downloads and compatibility note (#102)

Adds a badge showing downloads/month and a note that minor versions
before v1.0.0 may not be backwards compatible

Author: johnfraney

README.md

@@ -3,6 +3,7 @@
 [![Nox build status](https://github.com/blurry-dev/blurry/actions/workflows/github-actions-nox.yml/badge.svg?branch=main)](https://github.com/blurry-dev/blurry/actions/workflows/github-actions-nox.yml)
 [![PyPI package version](https://img.shields.io/pypi/v/blurry-cli.svg)](https://pypi.org/project/blurry-cli/)
 [![PyPI Python versions](https://img.shields.io/pypi/pyversions/blurry-cli.svg)](https://pypi.org/project/blurry-cli/)
+[![PyPI downloads per month](https://img.shields.io/pypi/dm/blurry-cli.svg)](https://pypi.python.org/pypi/blurry-cli/)
 ![Package license](https://img.shields.io/github/license/blurry-dev/blurry.svg)
 
 ![Blurry logo](docs/content/favicon.png)
@@ -16,6 +17,8 @@ Blurry also makes your images responsive, supports embedding Python source code
 
 Check out the docs and try it out!
 
+**Note**: Until v1.0.0, minor versions (0.x.0) may not be backwards compatible.
+
 ## Documentation
 
 View the documentation site at <https://blurry-docs.netlify.app/>.