Add docs on configuration changes

BREAKING_CHANGES_FOR_V10.md

@@ -2,7 +2,8 @@
 
 We've rewritten this library for v10, so that it provides all essential features for a Shopify app without depending on the [Active Resource](https://github.com/rails/activeresource) or [graphql-client](https://github.com/github/graphql-client) libraries.
 
-Here are the main features it provides:
+## New Features
+Here are the main features version 10 provides:
 
 - OAuth support, both with online and offline tokens.
 - Full, transparent support for JWT sessions for embedded apps and cookies for non-embedded ones.
@@ -13,10 +14,50 @@ Here are the main features it provides:
 - Webhook management, with features for adding handlers and registering them with Shopify.
 - Storefront GraphQL API support
 
-Please refer to [the documentation](docs/getting_started.md) in this repository for instructions on how to use each of these components.
+Please refer to the [Getting Started](docs/getting_started.md) guide in this repository for instructions on how to use each of these components.
 
+## Upgrade Guide
 With this, a lot changed in how apps access the library. Here are the updates you should make when migrating to v10:
 
+### 1. Configuration Changes
+#### Removed ShopifyAPI::Base
+`ShopifyAPI::Base` class has been removed. Previous versions of this gem used this class to configure API request setting like:
+  - API request version
+    - Previously: Set by `ShopifyAPI::Base.api_version = "xxxx"`
+    - Change: This is now configured in `api_version` of [ShopifyAPI::Context.setup](#shopifyapicontext-setup)
+  - Set `User-Agent` on API request header
+    - Previously: Set by `ShopifyAPI::Base.header["User-Agent"] = "xxxxx"`
+    - Change: This is now configured in `user_agent_prefix` of [ShopifyAPI::Context.setup](#shopifyapicontext-setup)
+  - Set custom headers on API requests
+    - Previously: Set by `ShopifyAPI::Base.header["User-Agent"] = "xxxxx"`
+    - Change: Custom headers can be added to requests when you use [`ShopifyAPI::Clients::HttpRequest`](https://github.com/Shopify/shopify-api-ruby/blob/main/lib/shopify_api/clients/http_request.rb#L14)
+
+
+#### ShopifyAPI::Context.setup
+Initializing the `ShopifyAPI::Context` with the parameters of your app by calling `ShopifyAPI::Context.setup` (example below) when your app starts (e.g `application.rb` in a Rails app).
+
+```ruby
+ShopifyAPI::Context.setup(
+  api_key: "<api-key>",
+  api_secret_key: "<api-secret-key>",
+  host_name: "<application-host-name>",
+  scope: "read_orders,read_products,etc",
+  is_embedded: true, # Set to true if you are building an embedded app
+  is_private: false, # Set to true if you are building a private app
+  api_version: "2021-01" # The version of the API you would like to use
+  user_agent_prefix: "<user_agent_prefix>" # Set a custom prefix for "User-Agent" header when making API requests
+  ###
+)
+```
+
+See other fields accepted during `ShopifyAPI::Context` setup in [context.rb](https://github.com/Shopify/shopify-api-ruby/blob/main/lib/shopify_api/context.rb).
+
+### Session Respository Changes
+
+### Client Changes
+#### GraphQL
+#### REST
+
 - Call `ShopifyAPI::Context.setup` when setting up your app. This class holds global configurations for your app and defines how the library behaves.
 Session persistence is handled by the [ShopifyApp](https://github.com/Shopify/shopify_app) gem and is recommended for use in the Rails context. See that gem's [documentation on how to use it](https://github.com/Shopify/shopify_app/blob/main/docs/shopify_app/sessions.md).
 - To change the `User-Agent` header, use `user_agent_prefix` in `ShopifyAPI::Context.setup`.