Added section in translator-docs for new script

docs/translator-docs.md

@@ -14,3 +14,8 @@ To translate BoxBuddy, please follow these instructions:
 - Open up `src/utils.rs` and find the comment which starts with `--TRANSLATORS:`
 - Comment out the if/else statement below by adding `//` to the start of the lines
 - Run `cargo run` as normal
+
+## Updating a `po` file from a new `pot` file
+
+To avoid having to manually rebase your `po` files each time the `pot` file changes (for example
+when all the line numbers change), you can simply run `make update-translations`.