Creating plugins docs WIP

docs/README.md

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+# Butterrobot Documentation
+
+## Index
+- [Creating a Plugin](./creating-a-plugin.md)

docs/creating-a-plugin.md

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+# Creating a Plugin
+
+## Example
+
+This simple "Marco Polo" plugin will answer _Polo_ to the user that say _Marco_:
+
+``` python
+# mypackage/plugins.py
+from butterrobot.plugins import Plugin
+from butterrobot.objects import Message
+
+
+class PingPlugin(Plugin):
+    name = "Marco/Polo"
+    id = "test.marco"
+
+    @classmethod
+    def on_message(cls, message, **kwargs):
+        if message.text == "Marco":
+            yield Message(
+                chat=message.chat, reply_to=message.id, text=f"polo",
+            )
+```
+
+``` python
+# setup.py
+# ...
+entrypoints = {
+    "test.marco" = "mypackage.plugins:MarcoPlugin"
+}
+
+setup(
+    # ...
+    entry_points=entrypoints,
+    # ...
+)
+```