Update HACKING.md and README.md

HACKING.md

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+# Blueprint files
+
+When adding a new Blueprint file in `data/res/`, remember to add it to both, the
+list of bluprint sources in `data/res/meson.build` and the list of resources in
+`data/res/musicus.gresource.xml` (`.blp` replaced with `.ui`).
+
+# Internationalization
+
+Execute the following commands from the project root directory to update
+translation files whenever translatable strings have been changed.
+
+1. Update `template.pot`
+
+    ```bash
+    xgettext \
+        --from-code=UTF-8 \
+        --add-comments \
+        --keyword=_ \
+        --keyword=C_:1c,2 \
+        --files-from=po/POTFILES \
+        --output=po/template.pot
+    ```
+
+2. Update translation files
+
+    ```bash
+    msgmerge \
+        --update \
+        --backup=off \
+        --no-fuzzy-matching \
+        po/de.po \
+        po/template.pot
+    ```