Adds upgrade guide to Bamboo 2.0 (#582)

What changed?
============

This adds an upgrade guide for migrating to Bamboo 2.0. At its core,
there are only two breaking changes: `bamboo_phoenix` being extracted
and the new usage of `Mailer.deliver_now` & `Mailer.deliver_later`
functions.

guides/upgrade_to_2_0.md

@@ -0,0 +1,105 @@
+Upgrading to Bamboo 2.0
+=======================
+
+Bamboo 2.0 ships with some breaking changes (hence the major version bump). But
+don't worry, if you love Bamboo just as it is, there's an easy upgrade path.
+
+There are two breaking changes:
+
+- `Bamboo.Phoenix` extracted to [bamboo_phoenix]
+- `Bamboo.Mailer.deliver_now/2` and `deliver_later/2` don't raise on errors
+
+Let's cover each in turn.
+
+## Breaking change: `Bamboo.Phoenix` extracted to `bamboo_phoenix`
+
+`Bamboo.Phoenix` has been extracted to the [bamboo_phoenix] library. If you use
+`Bamboo.Phoenix` to render your email templates, add `bamboo_phoenix` to your
+dependencies:
+
+```elixir
+defp deps do
+  [
+    ...
+    {:bamboo, "~> 2.0"},
+    {:bamboo_phoenix, "~> 1.0"}
+    ...
+  ]
+end
+```
+
+## Breaking change: `deliver_now/2`/`deliver_later/2` return `:ok` & `:error` tuples
+
+`Bamboo.Mailer`'s `deliver_now/2` and `deliver_later/2` no longer raise errors.
+Instead, they now return an `{:ok, email}` and `{:error, error}`, where the
+`error` is an exception struct or an error message. If you pass `response:
+true` as an argument, the return value will be `{:ok, email, response}`.
+
+If you prefer seeing code, this is the change in `@spec` signature for
+`deliver_now/2`:
+
+```diff
+-  @spec deliver_now(Bamboo.Email.t(), Enum.t()) :: Bamboo.Email.t() | {Bamboo.Email.t(), any}
++  @spec deliver_now(Bamboo.Email.t(), Enum.t()) ::
++          {:ok, Bamboo.Email.t()}
++          | {:ok, Bamboo.Email.t(), any}
++          | {:error, Exception.t() | String.t()}
+```
+
+Note that `deliver_later/2` will only return errors that happen _prior_ to
+scheduling the delivery of the email. What happens once the delivery is
+scheduled depends on what [delivery strategy] you are using.
+
+Those who want to handle errors on their own can now pattern match on the `:ok`
+and `:error` tuple responses. If you don't want to handle the errors and like
+how Bamboo behaves prior to 2.0, there's a simple upgrade path. 👇
+
+### Simple upgrade path
+
+`Bamboo.Mailer` comes with `deliver_now!/2` and `deliver_later!/2`. Those two
+functions mirror the behavior that `deliver_now/2` and `deliver_later/2` had
+before 2.0.
+
+Hopefully, that makes for a simple upgrade path for those who don't want to
+handle the `{:ok, email}` and `{:error, error}` tuples. You only need to
+change:
+
+- `deliver_now/2` to `deliver_now!/2`, and
+- `deliver_later/2` to `deliver_later!/2`
+
+Note that `deliver_later!/2` will only raise email validation errors _before_
+scheduling the email delivery. What happens after the delivery is scheduled
+depends on the [delivery strategy] you are using (e.g.
+`TaskSupervisorStrategy`).
+
+### `TaskSupervisorStrategy`
+
+Regardless of whether you use `deliver_later/2` or `deliver_later!/2`, if you
+use the `TaskSupervisorStrategy` for delivering emails, it will continue to
+raise errors when emails fail to be delivered.
+
+If you want control of those errors, you can implement a custom [delivery
+strategy], to handle errors coming from `adapter.deliver`.
+
+For now, `TaskSupervisorStrategy` continues to work as it did prior to `2.0`,
+so no change is needed here to upgrade to Bamboo 2.0.
+
+### Check with your adapter
+
+Each adapter needs to upgrade to satisfy the new [adapter.deliver callback].
+Check with your adapter to see if it supports the new `ok` and `error` tuple
+API before upgrading to Bamboo 2.0.
+
+If you use SendGrid, Mailgun, or Mandrill, your adapter is already updated with
+Bamboo 2.0.
+
+That's it! For a full list of changes, please refer to the [changelog].
+
+And if you find any issues with this upgrade guide, please let us know by
+[opening an issue] or submitting a pull-request.
+
+[adapter.deliver callback]: https://hexdocs.pm/bamboo/2.0.0/Bamboo.Adapter.html#c:deliver/2
+[delivery strategy]: https://hexdocs.pm/bamboo/2.0.0/Bamboo.DeliverLaterStrategy.html
+[opening an issue]: https://github.com/thoughtbot/bamboo/issues
+[bamboo_phoenix]: https://hexdocs.pm/bamboo_phoenix
+[changelog]: https://github.com/thoughtbot/bamboo/blob/master/CHANGELOG.md

mix.exs

@@ -19,7 +19,7 @@ defmodule Bamboo.Mixfile do
       build_embedded: Mix.env() == :prod,
       start_permanent: Mix.env() == :prod,
       package: package(),
-      docs: [main: "readme", extras: ["README.md"]],
+      docs: docs(),
       deps: deps(),
       xref: [exclude: [IEx]]
     ]
@@ -47,6 +47,16 @@ defmodule Bamboo.Mixfile do
   defp elixirc_paths(_), do: elixirc_paths()
   defp elixirc_paths, do: ["lib"]
 
+  defp docs do
+    [
+      main: "readme",
+      extras: [
+        "README.md",
+        "guides/upgrade_to_2_0.md"
+      ]
+    ]
+  end
+
   defp deps do
     [
       {:plug, "~> 1.0"},