Add READMEs for API content directories (#1895)

Co-authored-by: Rachael Sewell <rachmari@github.com>

README.md

@@ -50,6 +50,8 @@ There are a few more things to know when you're getting started with this repo:
 In addition to the README you're reading right now, this repo includes other READMEs that describe the purpose of each subdirectory in more detail:
 
 - [content/README.md](content/README.md)
+- [content/graphql/README.md](content/graphql/README.md)
+- [content/rest/README.md](content/rest/README.md)
 - [contributing/README.md](contributing/README.md)
 - [data/README.md](data/README.md)
 - [data/reusables/README.md](data/reusables/README.md)

content/graphql/README.md

@@ -0,0 +1,10 @@
+# GraphQL
+
+The `/content/graphql` directory is where the GitHub GraphQL API docs live!
+
+* The `/content/graphql/guides` and `/content/graphql/overview` directories contain articles that are human-editable.
+* The `/content/graphql/reference` directory contains an article for each GraphQL data type used in the GitHub GraphQL API. Most of the content in this directory is rendered using `include` tags.
+
+  The content rendered by `include` tags is sourced from the `/lib/graphql/static` directory, which is automatically generated from the API source code internally in GitHub, and should not be edited by a human. For more information, see the [`/lib/graphql/README.md`](/lib/graphql/README.md).
+
+  **As a result, we cannot accept contributions to GraphQL API reference content in this repository.**

content/rest/README.md

@@ -0,0 +1,10 @@
+# REST
+
+The `/content/rest` directory is where the GitHub REST API docs live!
+
+* The `/content/rest/guides` and `/content/rest/overview` directories contain regular articles. These are human-editable.
+* The `/content/rest/reference` directory contains an article for each group of endpoints in the GitHub REST API. Most of the content in this directory is rendered using `include` tags.
+
+  The content rendered by `include` tags is sourced from the `/lib/rest/static` directory, which is automatically generated from the API source code internally in GitHub, and should not be edited by a human. For more information, see the [`/lib/rest/README.md`](/lib/rest/README.md).
+
+  **As a result, we cannot accept contributions to REST API reference content in this repository.**