Add LSP extension documentation

Author: award999

Contributor Documentation/LSP Extensions.md

@@ -153,6 +153,58 @@ export interface DoccDocumentationResponse {
 }
 ```
 
+## `textDocument/playgrounds`
+
+New request for return the list of #Playground macro expansions in a given text document.
+
+Primarily designed to allow editors to provide a list of available playgrounds in the project workspace and allow
+jumping to the locations where the #Playground macro was expanded.
+
+The request parses a given text document and returns the location, identifier, and optional label when available
+for each #Playground macro expansion. The request is intended to be used in combination with the `workspace/playgrounds`
+request where the `workspace/playgrounds` provides the full list of playgrounds in the workspace and `textDocument/playgrounds`
+can be called after document changes. This way the editor can itself keep the list of playgrounds up to date without needing to
+call `workspace/playgrounds` each time a document is changed.
+
+SourceKit-LSP will advertise `textDocument/playgrounds` in its experimental server capabilities if it supports it.
+
+- params: `DocumentPlaygroundParams`
+- result: `PlaygroundItem[]`
+
+```ts
+export interface DocumentPlaygroundParams {
+  /**
+   * The document to parse for playgrounds.
+   */
+  textDocument: TextDocumentIdentifier;
+}
+/**
+ * A `PlaygroundItem` represents an expansion of the #Playground macro, providing the editor with the
+ * location of the playground and identifiers to allow executing the playground through a "swift play" command.
+ */
+export interface PlaygroundItem {
+  /**
+   * Unique identifier for the `PlaygroundItem`. Client can run the playground by executing `swift play <id>`.
+   * 
+   * This property is always present whether the `PlaygroundItem` has a `label` or not.
+   *
+   * Follows the format output by `swift play --list`.
+   */
+  id: string;
+
+  /**
+   * The label that can be used as a display name for the playground. This optional property is only available
+   * for named playgrounds. For example: `#Playground("hello") { print("Hello!) }` would have a `label` of `"hello"`.
+   */
+  label?: string
+
+  /**
+   * The location of the of where the #Playground macro expansion occured in the source code.
+   */
+  location: Location
+}
+```
+
 ## `textDocument/symbolInfo`
 
 New request for semantic information about the symbol at a given location.