Improve summary of the "Installers" section (en)

* remove duplicate information about ruby-build and ruby-install
  (they have dedicated sections by now)
* mention multiple installations
* add a note about RVM
Update Installation (ko)
Improve summary of the "Installers" section (ko)

* ref 24c27d2
Link to installation page from quickstart page (ko)

* ref c774ec1
Update Documentation (ko)
Add books link, Remove miss inputed lines (ko)
Translate 2014-11-25-ruby-2-2-0-preview2-released [vi]
Fix links and spacing (ko)
Merge pull request #916 from ruby-vietnam/translate-ruby-2-2-pr2

Translate 2014-11-25-ruby-2-2-0-preview2-released [vi]
Fix missing punctuation; rewrap (vi)
Merge pull request #913 from marocchino/ko-install

Update Installation (ko)