docs: name the registries behind assets and dedup

The guides said files over 50 MB become release assets and that RPG Maker and ScummVM were the trees kept out of dedup. .gitignore decides the first, four committed files are over 50 MB, and NODEDUP_DIRS holds eleven trees.

scripts/generate_readme.py

@@ -719,8 +719,10 @@ Full walkthrough: [profiling guide]({link("wiki/profiling")}).
 
 - `bios/Manufacturer/Console/filename` for canonical files
 - `bios/Manufacturer/Console/.variants/filename.sha1prefix` for alternate versions
-- Files >50 MB go in GitHub release assets (`large-files` release)
-- RPG Maker and ScummVM directories are excluded from deduplication
+- A file too large for git goes to the `large-files` GitHub release and its
+  path into `.gitignore`, the list that decides what is served from the release
+- Trees addressed by path are kept out of deduplication (`NODEDUP_DIRS` in
+  `scripts/dedup.py`)
 - Two paths differing only by case break clones on Windows and macOS,
   and `tests/test_no_case_collisions.py` enforces this