CB-7350 Improve README.md markdown

Also annotate points where I think the content is wrong
CB-7350 Rework createmobilespec

* Don't assume that the people running mobilespec are developers

* Correct __dirname path output

* quit instead of return in top level code...

* use joinable array for platform_layout, localPlatforms
  - this means the couldNotFind can report the correct repo name for bb10

* Default to not logging pushd/popd (--debug to get it back)

* Explain how to get coho
* Provide information about how to use coho to get the right things more or less at the right time
* Add could not find routine with coho instructions
* Fix instructions for getting cli+lib and plugins
* Remove recommendation to clone android
* a few lines earlier it already said to repo-clone -r plugins

* Skip broken platforms

* Refactor installPlugins, updateJS, summary

* warn about missing plugman
* fixes --plugman for non ios platforms
* add blackberry10 config dir info for plugman
* Adding fallback for createmobilespec --plugman config.xml
* correctly copy www directory contents...

* Depend on grunt-cli
  + report missing grunt

* Windows: fix symlink text

* Brand: JavaScript