[Analyzer] Add LSP dart.analysisExcludedFolders to readme

Change-Id: Iaa4cf8f9f2697c6fd3583018566f9201820ead8b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/173966
Auto-Submit: Danny Tuppeny <danny@tuppeny.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

Author: DanTup

pkg/analysis_server/tool/lsp_spec/README.md

@@ -29,6 +29,7 @@ Note: In LSP the client makes the first request so there is no obvious confirmat
 
 Client workspace settings are requested with `workspace/configuration` during initialization and re-requested whenever the client notifies the server with `workspace/didChangeConfiguration`. This allows the settings to take effect without restarting the server.
 
+- `dart.analysisExcludedFolders`: An array of paths (absolute or relative to each workspace folder) that should be excluded from analysis.
 - `dart.enableSdkFormatter`: When set to `false`, prevents registration (or unregisters) the SDK formatter. When set to `true` or not supplied, will register/reregister the SDK formatter.
 - `dart.lineLength`: The number of characters the formatter should wrap code at. If unspecified, code will be wrapped at `80` characters.
 - `dart.completeFunctionCalls`: Completes functions/methods with their required parameters.