Support top-level README and LICENSE at workspace root as defaults

[doriable]

Feature

Right now, in order to get a README and LICENSE to be pushed alongside a module, users need to symlink these files into their modules. This can be inconvenient.
With v2 modules, the buf.yaml configuration is designed to be at the top-level of a repository, alongside many of these common files. If a module does not have a README/buf.md and/or LICENSE explicitly defined and/or symlinked to the module root, should we support pushing the top-level files as a default?

I am proposing this for v2 buf.yaml workspaces only.

[bufdev]

I'd say yes overall. Readme is a closer call, but as long as module-specific readme takes precedence, then ok.

[doriable]

This has been addressed in #3206