doc: add translation strings policy

doc/README.md

@@ -55,6 +55,7 @@ The Bitcoin repo's [root README](https://github.com/bitcoin/bitcoin/blob/master/
 - [Release Process](release-process.md)
 - [Source Code Documentation (External Link)](https://dev.visucore.com/bitcoin/doxygen/)
 - [Translation Process](translation_process.md)
+- [Translation Strings Policy](translation_strings_policy.md)
 - [Unit Tests](unit-tests.md)
 - [Unauthenticated REST Interface](REST-interface.md)
 - [BIPS](bips.md)