Add table of contents and migration notesgi

README.md

@@ -18,6 +18,12 @@ A public GraphQL API for information about countries, continents, and languages.
 2. The `country.continent` and `country.languages` are now objects and arrays of objects, respectively.
 3. Each `Country` has an array of `states` populated by their states/provinces, if any.
 
+- [Writing queries](#writing-queries)
+  - [Migration notes (pre-March 2020)](#migration-notes-pre-march-2020)
+- [Docs](#docs)
+- [Examples](#examples)
+- [License](#license)
+
 ## Writing queries
 
 ```graphql
@@ -56,6 +62,20 @@ The above GraphQL query will produce the following JSON response:
 }
 ```
 
+### Migration notes (pre-March 2020)
+
+In the first version of this API, `code` arguments were treated as `String`s. Now, they are enums of valid options only. To migrate, simply remove the double quotes from your existing code arguments:
+
+```diff
+{
+- country(code: "CA") {
++ country(code: CA) {
+    name
+    emoji
+  }
+}
+```
+
 ## Docs
 
 Check out [the playground](https://countries.trevorblades.com) to explore the schema and test out some queries.