[V3] migration docs (#992)

* [Docs] start on migration guide

* [Docs] add more to migration docs

* [Docs] fix imports in examples

* [Docs] add migration guide to toctree

* [Docs] add a cog creation guide

* [Docs] expand on cog packages in migration guide

* Fix typo

* [Docs] add link for modlog docs

docs/guide_migration.rst

@@ -0,0 +1,56 @@
+.. V3 Migration Guide
+
+.. role:: python(code)
+    :language: python
+
+====================
+Migrating Cogs to V3
+====================
+
+First, be sure to read `discord.py's migration guide <http://discordpy.readthedocs.io/en/rewrite/migrating.html>`_
+as that covers all of the changes to discord.py that will affect the migration process
+
+----------------
+Red as a package
+----------------
+
+V3 makes Red a package that is installed with :code:`pip`. Please
+keep this in mind when writing cogs as this affects how imports 
+should be done (for example, to import :code:`pagify` in V2, one
+would do :code:`from .utils.chat_formatting import pagify`; in
+V3, this becomes :code:`from redbot.core.utils.chat_formatting import pagify`)
+
+----------------
+Cogs as packages
+----------------
+
+V3 makes cogs into packages. See `the cog creation guide </guide_cog_creation>`_
+for more on how to create packages for V3
+
+------
+Config
+------
+
+Config is V3's replacement for :code:`dataIO`. Instead of fiddling with
+creating config directories and config files as was done in V2, V3's
+Config handles that whilst allowing for easy storage of settings on a
+per-server/member/user/role/channel or global basis. Be sure to check
+out :doc:`/framework_config` for the API docs for Config as well as a
+tutorial on using Config.
+
+----
+Bank
+----
+
+Bank in V3 has been split out from Economy. V3 introduces the ability
+to have a global bank as well as the ability to change the bank name
+and the name of the currency. Be sure to checkout :doc:`/framework_bank`
+for more on Bank
+
+-------
+Mod Log
+-------
+
+V3 introduces Mod Log as an API, thus allowing for cogs to add custom case
+types that will appear in a server's mod log channel. Be sure to checkout
+:doc:`/framework_modlog` for more on Mod Log`