Create source/posts directory to manage posts(articles) file.
Update README with creating source/posts directory.
Merge branch 'develop' of github.com:wantedly/wantedly.github.io into spesnova/change-source-layout

Conflicts:
	README.md
Merge pull request #19 from wantedly/spesnova/change-source-layout

記事ディレクトリを追加