docs: Add conda-forge install details to README (#82)

* Add conda-forge badge to README.
   - c.f. https://github.com/conda-forge/pyhepmc-feedstock
* Add conda-forge install instructions to README.

CONTRIBUTING.md

@@ -44,7 +44,7 @@ python3 -m venv venv
 Activate the virtualenv and install the required packages for development:
 ```
 . venv/bin/activate
-pip install -v -e .'[test]'
+python -m pip install -v -e .'[test]'
 ```
 This builds the package in develop mode and installs the extra libraries used for testing. The build should work fine if you have a reasonably recent C++ compiler, since pyhepmc is continuously tested on against gcc, clang and msvc. If it does not, please submit an issue with the build log and your compiler version.
 
@@ -60,6 +60,6 @@ To leave the virtualenv, call `deactivate` or close the shell.
 
 If you want to use your local version for productive work, install it from the local project folder:
 ```
-pip install --upgrade <project folder>
+python -m pip install --upgrade <project folder>
 ```
 The `--upgrade` option makes sure that an already existing pyhepmc version is replaced.

README.rst

@@ -9,6 +9,9 @@ A Pythonic wrapper for the `HepMC3 C++ library <http://hepmc.web.cern.ch/hepmc>`
 .. image:: https://badge.fury.io/py/pyhepmc.svg
   :target: https://pypi.org/project/pyhepmc
 
+.. image:: https://img.shields.io/conda/vn/conda-forge/pyhepmc.svg
+  :target: https://github.com/conda-forge/pyhepmc-feedstock
+
 .. image:: https://coveralls.io/repos/github/scikit-hep/pyhepmc/badge.svg?branch=main
   :target: https://coveralls.io/github/scikit-hep/pyhepmc?branch=main
 
@@ -21,9 +24,21 @@ HepMC3 has its own Python bindings. Why should you use these?
 
 **pyhepmc is easy to install**
 
-The command ``pip install pyhepmc`` just works on all common platforms. Since we publish binary wheels, you don't need to compile anything. Since we include the HepMC3 library, you don't need to install it separately either.
+The command
+
+.. code:: bash
+
+   python -m pip install pyhepmc
+
+just works on all common platforms. Since we publish binary wheels, you don't need to compile anything. Since we include the HepMC3 library, you don't need to install it separately either.
+
+However, building from source is also easy. External software is not required. Just download the repository with ``git clone --recursive`` and run ``python -m pip install -v -e .``.
+
+``pyhepmc`` is also available on conda-forge
+
+.. code:: bash
 
-However, building from source is also easy. External software is not required. Just download the repository with ``git clone --recursive`` and run ``pip install -v -e .``.
+   conda install --channel conda-forge pyhepmc
 
 **pyhepmc is Pythonic, Numpy-friendy, and Jupyter notebook-friendly**