diff --git a/contrib/deterministic-build/README.md b/contrib/deterministic-build/README.md
new file mode 100644
index 000000000..bff073b02
--- /dev/null
+++ b/contrib/deterministic-build/README.md
@@ -0,0 +1,12 @@
+# Notes
+
+The frozen dependency lists in this folder are *generated* files.
+
+- Starting from `contrib/requirements/requirements*.txt`,
+- we use the `contrib/freeze_packages.sh` script,
+- to generate `contrib/deterministic-build/requirements*.txt`.
+
+The source files list direct dependencies with loose version requirements,
+while the output files list all transitive dependencies with exact version+hash pins.
+
+The build scripts only use these hash pinned requirement files.
diff --git a/contrib/requirements/requirements-build-android.txt b/contrib/requirements/requirements-build-android.txt
index 2f579ede2..a845fd510 100644
--- a/contrib/requirements/requirements-build-android.txt
+++ b/contrib/requirements/requirements-build-android.txt
@@ -16,3 +16,7 @@ six
 sh>=1.10
 pep517<0.7.0
 toml
+
+# needed for the Qt/QML Android GUI:
+PyQt-builder
+typing-extensions