add skip-worktree instructions

Author: gtm-nayan

packages/playground/README.md

@@ -0,0 +1,5 @@
+You may use this package to experiment with your changes to Svelte.
+
+To prevent any changes you make from being committed, run `git update-index --skip-worktree ./**/*.*` in this directory.
+
+If you would actually like to make some changes to the files here for everyone then run `git update-index --no-skip-worktree ./**/*.*` before committing.