libreoffice: improve nixpkgs' README update instructions

pkgs/applications/office/libreoffice/README.md

@@ -1,9 +1,19 @@
 LibreOffice
 ===========
 
-To generate `src-$VARIANT/download.nix`, i.e. list of additional sources that
-the libreoffice build process needs to download:
+Here are instructions how to update libreoffice. In the below commands and
+instructions, $VARIANT is either `still` or `fresh`.
 
-    nix-shell gen-shell.nix --argstr variant VARIANT --run generate
+1. Update the necessary version parts in `src-$VARIANT/primary.nix`.
+2. Run the following command:
 
-Where VARIANT is either `still` or `fresh`.
+```bash
+nix-shell gen-shell.nix --argstr variant VARIANT --run generate
+```
+
+3. It will fail initially, due to hash mismatch in the main `src` attribute.
+   Fix the hash according to the failure.
+4. Run the command from step 2 again, now it will take a while.
+5. Zero-out the hashes of the translations and help attributes in
+   `src-$VARIANT/primary.nix`.
+6. Update the hashes above by building `libreoffice-$VARIANT`.