(sdks) Document customSections parameter for SDK README customization (#830)

Co-authored-by: promptless[bot] <179508745+promptless[bot]@users.noreply.github.com>
Co-authored-by: Devin Logan <[email protected]>

Author: promptless[bot]

fern/products/cli-api-reference/cli-changelog/2025-09-22.mdx

@@ -7,7 +7,7 @@
 
 
 ## 0.78.4
-**`(fix):`** Collapse any combination of optional and nullable to optional<nullable<T>>.
+**`(fix):`** Collapse any combination of optional and nullable to `optional<nullable<T>>`.
 
 ## 0.78.3
 **`(chore):`** Support IR v59 in the Rust SDK generator.

fern/products/sdks/snippets/readme-options.mdx

@@ -10,6 +10,19 @@ readme:
     method: "POST"
     path: "/users"
     stream: false
+  customSections:
+    - title: "Custom Section"
+      language: "java"
+      content: |
+        This is a custom section. Latest package info is {{ group }}:{{ artifact }}:{{ version }}.
+    - title: "Custom Section"
+      language: "typescript"
+      content: |
+        Custom section for {{ packageName }}
+    - title: "Another Custom Section"
+      language: "typescript"
+      content: |
+        A second custom section for {{ packageName }}
   features:
     authentication:
       - method: "POST"
@@ -42,10 +55,6 @@ readme:
   Sections to disable in the README. Supported values: `"contributing"`.
 </ParamField>
 
-<ParamField path="defaultEndpoint" type="ReadmeEndpointSchema" required={false} toc={true}>
-  Specifies which endpoint's code snippet to showcase as the primary example in the README.
-</ParamField>
-
 <ParamField path="features" type="map<string, list<ReadmeEndpointSchema>>" required={false} toc={true}>
   Organizes endpoints into named feature sections within the README. Each feature creates a dedicated section with example code snippets for the specified endpoints.
 </ParamField>
@@ -64,4 +73,37 @@ Specifies which endpoint's code snippet to showcase as the primary example in th
 
 <ParamField path="defaultEndpoint.stream" type="boolean" required={false} default="false" toc={true}>
   Whether the endpoint is a streaming endpoint. Defaults to `false`.
+</ParamField>
+
+### Custom sections
+
+Define a custom section in the generated README for a specific SDK. 
+
+<ParamField path="customSections.title" type="string" required={true} toc={true}>
+  The title of the custom section as it will appear in the README.
+</ParamField>
+<ParamField path="customSections.language" type="'java' | 'typescript'" required={true} toc={true}>
+  The target SDK language for this section. The custom section will only appear in README files generated for the specified language.
+</ParamField>
+<ParamField path="customSections.content" type="string" required={true} toc={true}>
+  The Markdown content of the custom section. You can use template variables in the format `{{ variable }}` that will be dynamically replaced with values specific to each SDK language when the README is generated.
+
+  Available template variables by language:
+
+  | Language | Variable | Description |
+  |----------|----------|-------------|
+  | TypeScript | `packageName` | Name of your package, as specified in the [`package-name` field](/sdks/generators/typescript/configuration#package-name) |
+  | Python | `packageName` | Name of your package, as specified in the [`package-name` field](/sdks/generators/python/configuration#package-name) |
+  | Go | `owner` | The owner of your Go module |
+  | Go | `repo` | The [repository](/sdks/generators/go/publishing#configure-output-location) where your Go module is published |
+  | Go | `version` | SDK version |
+  | Java | `group` | Maven `groupId` [from `coordinate` field](/sdks/generators/java/publishing#configure-maven-coordinate) |
+  | Java | `artifact` | Maven `artifactId` [from `coordinate` field](/sdks/generators/java/publishing#configure-maven-coordinate) |
+  | Java | `version` | SDK version |
+  | C#/.NET | `packageName` | Name of your package, as specified in the [`package-name` field](/sdks/generators/csharp/configuration#package-name) |
+  | PHP | `packageName` | Name of your package, as specified in the [`package-name` field](/sdks/generators/php/configuration#package-name) |
+  | Ruby | `packageName` | Name of your package, as specified in the [`package-name` field](/sdks/generators/ruby/configuration#package-name) |
+  | Swift | `gitUrl` |  The [URL](/sdks/generators/swift/publishing#verify-package-availability) where your Swift package is published. For example, `https://github.com/fern-api/basic-swift-sdk` |
+  | Swift | `minVersion` | SDK version |
+
 </ParamField>