docs: add -w flag to pnpm example (#9160)

It's suggested to install this into the repository root. If you do this,
you must use `--workspace-root` flag or you will get this error:


>  ERR_PNPM_ADDING_TO_ROOT  Running this command will add the dependency
to the workspace root, which might not be what you want - if you really
meant it, make it explicit by running this command again with the -w
flag (or --workspace-root). If you don't want to see this warning
anymore, you may set the ignore-workspace-root-check setting to true.

docs/repo-docs/getting-started/add-to-existing-repository.mdx

@@ -69,7 +69,7 @@ We recommend you install `turbo` both globally and into your repository's root f
     # Global install
     pnpm add turbo --global
     # Install in repository
-    pnpm add turbo --save-dev
+    pnpm add turbo --save-dev --workspace-root
     ```
 
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