Binder integration via jupytext (PlasmaPy#656)

Adds Binder integration via jupytext, bypassing RTD, as per https://jupytext.readthedocs.io/en/latest/formats.html#sphinx-gallery-scripts

.jupyter/jupyter_notebook_config.py

@@ -0,0 +1,3 @@
+c.NotebookApp.contents_manager_class = "jupytext.TextFileContentsManager"
+c.ContentsManager.preferred_jupytext_formats_read = "py:sphinx"
+c.ContentsManager.sphinx_convert_rst2md = True

README.md

@@ -10,6 +10,7 @@
 [![Build status](https://ci.appveyor.com/api/projects/status/hbduy62sqrvy8rn7?svg=true)](https://ci.appveyor.com/project/namurphy/plasmapy)
 [![codecov](https://codecov.io/gh/PlasmaPy/PlasmaPy/branch/master/graph/badge.svg)](https://codecov.io/gh/PlasmaPy/PlasmaPy)
 [![Documentation Status](https://readthedocs.org/projects/plasmapy/badge/?version=latest)](http://plasmapy.readthedocs.io/en/latest/?badge=latest)
+[![Binder](https://mybinder.org/badge_logo.svg)](https://mybinder.org/v2/gh/PlasmaPy/PlasmaPy/master?filepath=plasmapy%2Fexamples)
 
 [![Matrix](https://matrix.to/img/matrix-badge.svg)](https://riot.im/app/#/room/#plasmapy:matrix.org)
 [![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/PlasmaPy/Lobby)

binder/requirements.txt

@@ -0,0 +1,4 @@
+-r ../requirements/automated-documentation-tests.txt
+pytest
+jupytext
+.

changelog/656.trivial.rst

@@ -0,0 +1,2 @@
+Allow running our sphinx-gallery examples as Jupyter notebooks via Binder
+

plasmapy/examples/README.txt

@@ -9,4 +9,6 @@ Examples
 General examples
 ----------------
 
-General-purpose and introductory examples from PlasmaPy
+General-purpose and introductory examples from PlasmaPy.
+
+`You may also run these online via Binder (link uses the current master version). <https://mybinder.org/v2/gh/PlasmaPy/PlasmaPy/master?filepath=plasmapy%2Fexamples>`_