Add link to Japanese translation

Author: adamgreig

src/preface.md

@@ -33,6 +33,14 @@ This book mainly targets to two audiences:
 [`msp430-rt`]: https://crates.io/crates/msp430-rt
 [`riscv-rt`]: https://crates.io/crates/riscv-rt
 
+## Translations
+
+This book has been translated by generous volunteers. If you would like your
+translation listed here, please open a PR to add it.
+
+* [Japanese](https://tomoyuki-nakabayashi.github.io/embedonomicon/)
+  ([repository](https://github.com/tomoyuki-nakabayashi/embedonomicon))
+
 ## Requirements
 
 This book is self contained. The reader doesn't need to be familiar with the