• Have you followed the contributing guidelines?
• Have you explained what your changes do, and why they add value to the Guides?

Please note: we will close your PR without comment if you do not check the boxes above and provide ALL requested information.

This pull request is an on-going effort to provide the Japanese translation.

Translation guidelines

• Add single space before and after a word which won't be translated into Japanese like proper nouns.

TODO

• Add space before and after English word.
  • best-practices.md
  • building-community.md
  • code-of-conduct.md
  • finding-users.md
  • getting-paid.md
  • how-to-contribute.md
  • leadership-and-governance.md
  • legal.md
  • metrics.md
  • starting-a-project.md
• Confirm there are no untranslated sentence.
  • best-practices.md
  • building-community.md
  • code-of-conduct.md
  • finding-users.md
  • getting-paid.md
  • how-to-contribute.md
  • leadership-and-governance.md
  • legal.md
  • metrics.md
  • starting-a-project.md
• Self review by @yuichielectric
  • best-practices.md
  • building-community.md
  • code-of-conduct.md
  • finding-users.md
  • getting-paid.md
  • how-to-contribute.md
  • leadership-and-governance.md
  • legal.md
  • metrics.md
  • starting-a-project.md
• Replace 「貢献」 to 「コントリビュート」