docs: include --workspace-root on pnpm add command (#2753)
Given Turborepo is intended to use on workspaces, it is very likely users are adding it to the root of an already configured workspace.
If you run `pnpm add` on a workspace root, you get an 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.
```
This adds the `--workspace-root` flag to the docs command so this is skipped.
This might also be needed for yarn but I'm not sure if they have a stable option besides `--ignore-workspace-root-check` on 1.x.
