Update contributing guide to specify firebase tools version

[bidoubiwa]

Since the latest version of firebase-tools v11.x.x the emulator has been removed.

By enforcing the version number of the firebase-tool we ensure we are still able to emulate a local environment with the extension.

ps: I made the same change here, hope it avoids you wasting time finding the issue :)

[Haroenv]

I think they may have changed the command needed, as there's still extension emulation code: https://github.com/firebase/firebase-tools/tree/v11.0.0/src/emulator/extensions

Thanks for this PR already though!

[bidoubiwa]

I may have done something wrong at one point. I tried firebase emulator:start but did not manage to make it work the way the current emulator works. The documentation on emulating extensions is not up to date (you need access to this doc).

The way it works now is that you need to emulate it on an existing project.

You can install your extension in an actual Firebase project. We recommend using a test project for your testing. Use this testing workflow if you want to test your extension's end-to-end flow

I'll be watching this repo's contributing guide in case you find how to make it work!

[Haroenv]

I've got access to that documentation, but it mentions I can't share it publicly, you can send me an email (I couldn't find your address) for a version of the documentation :)