Added migirate docs

Author: eunwoo1104

docs/index.rst

@@ -27,6 +27,7 @@ discussion on GitHub!
 
    quickstart.rst
    gettingstarted.rst
+   migrate_to_109.rst
    discord_slash.rst
    events.rst
    discord_slash.utils.rst

docs/migrate_to_109.rst

@@ -0,0 +1,27 @@
+Migrate To 1.0.9
+================
+
+1.0.9 Update includes many feature add/rewrite for easier and better usage.
+However, due to that, it includes many critical breaking changes.
+
+This page will show how to deal with those changes.
+
+SlashContext.send()
+********************
+
+Before:
+
+.. code-block:: python
+
+    await ctx.send(4, content="Hello, World! This is initial message.")
+    await ctx.delete()
+    await ctx.send(content="This is followup message.")
+
+After:
+
+.. code-block:: python
+
+    await ctx.respond(eat=False)
+    msg = await ctx.send("Hello, World! This is initial message.")
+    await msg.delete()
+    await ctx.send("This is followup message.")