Add an example plugin project to source tree

example-plugin/src/flake8_example_plugin/on_by_default.py

@@ -0,0 +1,15 @@
+"""Our first example plugin."""
+
+
+class ExampleOne(object):
+    """First Example Plugin."""
+    name = 'on-by-default-example-plugin'
+    version = '1.0.0'
+
+    def __init__(self, tree):
+        self.tree = tree
+
+    def run(self):
+        """Do nothing."""
+        for message in []:
+            yield message