Japanese 🇯🇵 Translation

Hi, I just came across this repo and would love to contribute to the Japanese translation!

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EDIT: My friendly co-worker @kawamataryo kindly volunteered to work on the translation together! 🎉
Right now, we are working on a forked branch on my account here: https://github.com/yuichkun/javascript-testing-best-practices/blob/master/readme-jp.md

## Progress

- [x] Introduction
- [x] Section 0
- [ ] Section 1
- [x] 1.1
- [x] 1.2
- [x] 1.3
- [x] 1.4
- [x] 1.5
- [x] 1.6
- [x] 1.7
- [ ] 1.8
- [ ] 1.9
- [ ] 1.10
- [ ] 1.11
- [ ] 1.12
- [ ] 1.13
- [ ] Section 2
- [ ] 2.1
- [ ] 2.2
- [ ] 2.3
- [ ] 2.4
- [ ] 2.5
- [ ] 2.6
- [ ] 2.7
- [ ] Section 3
- [ ] 3.1
- [ ] 3.2
- [ ] 3.3
- [ ] 3.4
- [ ] 3.5
- [ ] 3.6
- [ ] 3.7
- [ ] 3.8
- [ ] 3.9
- [ ] 3.10
- [ ] 3.11
- [ ] Section 4
- [ ] 4.1
- [ ] 4.2
- [ ] 4.3
- [ ] 4.4
- [x] Section 5
- [x] 5.1
- [x] 5.2
- [x] 5.3
- [x] 5.4
- [x] 5.5
- [x] 5.6
- [x] 5.7
- [x] 5.8
- [x] 5.9
- [ ] Team