LB3 logo is used when creating LB4 app

[dhmlau]

Now that we have a new logo for LB4 (https://strongloop.com/strongblog/thanks-loopback-4-logo/), we should be using it when generating the app.

Current behavior

Currently, the README.md generated by the CLI tool is pointing to the LB3 logo (the green logo). It is coming from this file: https://github.com/strongloop/loopback-next/blob/master/packages/cli/generators/project/templates/README.md.

[![LoopBack](http://loopback.io/images/overview/powered-by-LB-xs.png)](http://loopback.io/)

Expected result

If the README will have a LB logo, it should be the new logo.

cc @virkt25

[virkt25]

Files attached if someone else takes on the task. I would recommend using the transparent-bg file. Can / should this be marked MVP + good first issue?

[virkt25]

I just realized we need a "Powered by LoopBack" version of the Logo. I've requested it from Cody and we should have that early next week.