Improving instructions for generating bundle from local directory

Referring to instructions at https://github.com/WICG/webpackage/blob/main/go/bundle/README.md#from-a-local-directory (Usage -> From a local directory section)

Bundles are either standalone for opening in a browser as a page, or used in subresource loading with WebBundles (https://chromestatus.com/feature/5710618575241216).
The latter is accomplished by omitting the -baseURL flag. It is correctly stated this makes the bundle relative, but it is not clear it also means such a bundle can't be opened as a page (I usually prefer relative paths over absolute ones).

See related discussion here https://bugs.chromium.org/p/chromium/issues/detail?id=1352933#c3

So I propose:
The -baseURL flag is mandatory for creating standalone bundles, which can be opened in a browser. The value can be arbitrary, but keep in mind it becomes the part of the URL shown in the address bar. It has to contain the trailing slash.
Skip -baseURL for bundles used in subresource loading.

Open go/bundle/README.md and read the Usage -> From a local directory section, along with the linked WebBundles context. Update the instructions to distinguish standalone bundles, which require a trailing-slash -baseURL, from bundles for subresource loading, which omit it; done means the two uses and browser-opening limitation are explicit.