📝 Refine readme upgrade section (#304)

Adds a small paragraph directly under the upgrade install command that mentions replacing existing usage with new usage. 

Also rewords the breaking changes intro since the recommended upgrade path is to just replace your existing usage. In addition to the rewording, the command change is also listed

README.md

@@ -138,8 +138,8 @@ specific options are supported here as well.
 ## Upgrading
 
 Prior versions of the Storybook SDK were drastically different than the current version. The
-command, it's args, and how the SDK works internally have changed completely. Using the old command
-will now result in a deprecation message. The new command is now integrated into
+command, it's arguments, and how the SDK works internally have changed completely. Using the old
+command with new versions will now result in an error message. The new command is now integrated into
 [`@percy/cli`](https://github.com/percy/cli) as a plugin.
 
 To use new versions of this SDK, you will have to also install the CLI with the SDK:
@@ -148,10 +148,16 @@ To use new versions of this SDK, you will have to also install the CLI with the
 $ npm install --save-dev @percy/cli @percy/storybook@next
 ```
 
+Since both the command and arguments have changed, you'll need to replace your existing usage with
+the [new usage described above](#usage). For some projects, this may require setting additional 
+[configuration options](#configuration). See the list of breaking changes below for details.
+
 ### Breaking changes
 
-There are a lot of breaking changes. So to upgrade, make sure you read through them. For one,
-previous flags are no longer available. Here are each of their alternatives:
+Most importantly, the command itself has changed and all previous arguments are no longer accepted.
+  
+- The `percy-storybook` command has been replaced with a [`percy` CLI](https://github.com/percy/cli) 
+  subcommand, `percy storybook`.
 
 - The previous `--build_dir` flag is now a command argument and there is no default build directory. If
   you relied on the default, it must now be explicitly provided.