Update README.md to availability on conda-forge (#76)

This PR

- Adds conda-forge instructions to the installation section
- Adds version badges for pypi and conda-forge (useful indicator that
  conda-forge update hasn't been approved)

README.md

@@ -5,13 +5,23 @@
 [![Tests Status](https://github.com/typeddjango/pytest-mypy-plugins/actions/workflows/test.yml/badge.svg)](https://github.com/typeddjango/pytest-mypy-plugins/actions/workflows/test.yml)
 [![Checked with mypy](http://www.mypy-lang.org/static/mypy_badge.svg)](http://mypy-lang.org/)
 [![Gitter](https://badges.gitter.im/mypy-django/Lobby.svg)](https://gitter.im/mypy-django/Lobby)
+[![PyPI](https://img.shields.io/pypi/v/pytest-mypy-plugins?color=blue)](https://pypi.org/project/pytest-mypy-plugins/)
+[![Conda Version](https://img.shields.io/conda/vn/conda-forge/pytest-mypy-plugins.svg?color=blue)](https://anaconda.org/conda-forge/pytest-mypy-plugins)
 
 ## Installation
 
+This package is available on [PyPI](https://pypi.org/project/pytest-mypy-plugins/)
+
 ```bash
 pip install pytest-mypy-plugins
 ```
 
+and [conda-forge](https://anaconda.org/conda-forge/pytest-mypy-plugins)
+
+```bash
+conda install -c conda-forge pytest-mypy-plugins
+```
+
 ## Usage
 
 ### Running