docs(installation): add explanation and better vocabulary

Signed-off-by: Édouard Lopez <dev+git@edouard-lopez.com>
docs(yo-generators-list): remote 'generator-' prefix

Signed-off-by: Édouard Lopez <dev+git@edouard-lopez.com>
style(readme,output): align output description

Signed-off-by: Édouard Lopez <dev+git@edouard-lopez.com>
docs(alias): reduce section header ambiguity

Signed-off-by: Édouard Lopez <dev+git@edouard-lopez.com>
docs(yo): details mechanism about suggestion construction

1. global generators: a list is _build at shell start-up_.
   So the list might be incomplete till the listing is finish
   (take few seconds).
2. local generators: are available when _current directory is a node
   project_. This means `npm root` give a path to a `nodes_modules/`
   that must exists.

Signed-off-by: Édouard Lopez <dev+git@edouard-lopez.com>