build: follow-up source-only tarballs

contrib/build-linux/sdist/README.md

@@ -1,5 +1,4 @@
-Source tarballs
-===============
+# Source tarballs
 
 ✓ _This file should be reproducible, meaning you should be able to generate
    distributables that match the official releases._
@@ -7,18 +6,31 @@ Source tarballs
 This assumes an Ubuntu (x86_64) host, but it should not be too hard to adapt to another
 similar system.
 
+We distribute two tarballs, a "normal" one (the default, recommended for users),
+and a strictly source-only one (for Linux distro packagers).
+The normal tarball, in addition to including everything from
+the source-only one, also includes:
+- compiled (`.mo`) locale files (in addition to source `.po` locale files)
+- compiled (`_pb2.py`) protobuf files (in addition to source `.proto` files)
+- the `packages/` folder containing source-only pure-python runtime dependencies
+
+
+## Build steps
+
 1. Install Docker
 
     See `contrib/docker_notes.md`.
 
-2. Build source tarball
+2. Build tarball
 
+    (set envvar `OMIT_UNCLEAN_FILES=1` to build the "source-only" tarball)
     ```
     $ ./build.sh
     ```
     If you want reproducibility, try instead e.g.:
     ```
     $ ELECBUILD_COMMIT=HEAD ELECBUILD_NOCACHE=1 ./build.sh
+    $ ELECBUILD_COMMIT=HEAD ELECBUILD_NOCACHE=1 OMIT_UNCLEAN_FILES=1 ./build.sh
     ```
 
 3. The generated distributables are in `./dist`.