Contributing.asciidoc: fix developer's guide link

The link directed to WeeChat.org instead of the developer's guide.
Merge pull request #37 from Mkaysi/patch-1

core: fix developer's guide link in Contributing.asciidoc
core: update Japanese translations
Merge pull request #38 from l/translation_ja

core: update Japanese translations
doc: update Japanese auto-generated files
README.asciidoc: fix links.