Add note about why `rustfmt.toml` is not a symlink

Other AOSP repositories use a symlink back to
`soong/scripts/rustfmt.toml`, but this repositories needs to be
self-contained.

Test: comment change only
Bug: 229825251
Change-Id: Ia050733eaa2a4fe6aec2024b5c4fd5bb1d131327

rustfmt.toml

 # Android Format Style
+#
+# Note that this is not a symlink back to soong/scripts/rustfmt.toml
+# since the ChromeOS team imports this repository. See
+# https://r.android.com/1998210 for details.
 
 edition = "2021"
 use_small_heuristics = "Max"