Added npm install command

[ParaSpl01t]

A shorter and easy to remember install command with possible version control.

[afdev82]

Yes, but to do that we should change the name of the plugin, otherwise I get an error when trying to restore the project with cordova prepare:

Failed to restore plugin "org.apache.cordova.shell-exec" from config.xml. You might need to try adding it again. Error: Error: npm: Command failed with exit code 1 Error output:
npm ERR! code E404
npm ERR! 404 Not found : org.apache.cordova.shell-exec
npm ERR! 404 
npm ERR! 404  'org.apache.cordova.shell-exec' is not in the npm registry.
npm ERR! 404 You should bug the author to publish it (or use the name yourself!)
npm ERR! 404 
npm ERR! 404 Note that you can also install from a
npm ERR! 404 tarball, folder, http url, or git url.

[afdev82]

In fact I found that also the way to install it described in the README has this problem too...
I needed to change manually "org.apache.cordova.shell-exec": {} with "cordova-plugin-shell-exec": {} in the package.json to get it work.
I think it would be nice to change the id in the plugin.xml to work with the latest cordova versions.