Add CLI docs for generating example and extension

[dhmlau]

From the official documentation (http://loopback.io/doc/en/lb4/Command-line-interface.html), it only has 2 commands, i.e. to generate application and controller. Since we have 2 more commands for generating example and extensions, we need to add it to the docs.

Also, the README (https://github.com/strongloop/loopback-next/tree/master/packages/cli) is out of date.

Acceptance Criteria

Update the documentation to include all the commands we support:

• lb4 controller

Places to update:

• official doc - http://loopback.io/doc/en/lb4/Command-line-interface.html
• @loopback/cli README

[shimks]

lb4 example is covered by this PR loopbackio/loopback.io#614

[dhmlau]

thanks @shimks . updated the description.

[dhmlau]

looks like lb4 controller is here as well.. closing this issue now.