Update translations and add notes

po/NOTES.md

@@ -0,0 +1,21 @@
+All commands should be executed from this directory!
+
+Regenerate `POTFILES.in` using:
+
+```bash
+find ../crates \( -name \*.rs -o -name \*.ui \) -print > POTFILES.in
+```
+
+Update `musicus.pot` using:
+
+```bash
+xgettext -f POTFILES.in -o musicus.pot
+```
+
+Update the translation files using e.g.:
+
+```bash
+msgmerge de.po musicus.pot > tmp.po
+# Inspect tmp.po for errors.
+mv tmp.po de.po
+```