Usage instructions not correct

### Expected Behaviour

Adobe I/O plugins should be installed as follows:

```sh-session
$ aio plugins:install @adobe/aio-cli-plugin-aem
$ aio aem:upload --help
USAGE
$ aio aem:upload -h http://myaeminstance -c admin:12345 myimage.jpg
...
```

This how other Adobe I/O CLI plugins are used.

### Actual Behaviour

The README provides these usage instructions. These work, but not correct as noted above.

```sh-session
$ npm install -g @adobe/aio-cli-plugin-aem
$ aio-aem COMMAND
running command...
$ aio-aem (-v|--version|version)
@adobe/aio-cli-plugin-aem/1.0.3 darwin-x64 node-v10.16.3
$ aio-aem --help [COMMAND]
USAGE
$ aio-aem COMMAND
...
```

Open the README and compare its current installation and command examples with the expected Adobe I/O CLI usage in the issue. Update the instructions to use the aio plugin installation and aio aem commands, and confirm the README no longer presents the old aio-aem workflow.

Written by the indexing model from the issue text.