Update to Installing Zend Tool using Composer

I have updated the docs for installing Zend Tool using Composer. I had to do a couple more steps that are not listed in the docs. I have also updated the example command shown. Is this OK? Let me know if it is not and I will make amends. Thanks.
NOTE: please delete my last pull request, it is wrong -sorry.
#1137 - removing advice about `$PATH`, as it only makes installation harder to understand for non-cli users
Merge branch 'hotfix/#1137-update-composer-installation-command'

Close #1137