Add sample project for upgradeable proxy usage

[zoeyTM]

resolves #641

[jkirkpatrick]

OZ upgradable contracts should not accept constructor parameters... How should I pass parameters when using a custom initializer? How do you specify the initializer or upgrade functions, e.g. updateToAndCall?

[kanej]

I split these two calls apart @zoeyTM

[kanej]

I did a run through locally.
I pushed a few changes, specifically:

1. splitting up the commands in the readme
2. including the example in the pnpm workspace (this commit)
3. Removing unused variables in the test file

I think all that is left is to rebase on development and bump the version of the example to the latest 0.15.5.

[zoeyTM]

@kanej merging because it's already rebased and it doesn't look like we update the versions of the example projects. We can update them later all at once if we want.

I also tried adding the ENS project to the workspace, but it's still bugged. I briefly tried to fix it, but it seems like a much more involved dive is needed and it's not worth the time right now.