cgitrc.5: add local tar signature example

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

cgitrc.5.txt

@@ -781,8 +781,19 @@ add a signature to a .tar.xz archive:
 
 If it is instead desirable to attach a signature of the underlying .tar, this
 will be linked, as a special case, beside a .tar.* link that does not have its
-own signature.
+own signature. For example, a signature of a tarball of the latest tag might
+be added with a similar command:
 
+    tag="$(git describe --abbrev=0)"
+    git notes --ref=refs/notes/signatures/tar add -C "$(
+        git archive --format tar --prefix "cgit-${tag#v}/" "$tag" |
+        gpg --output - --armor --detach-sign |
+        git hash-object -w --stdin
+    )" "$tag"
+
+Since git-archive(1) is expected to produce stable output between versions,
+this allows one to generate a long-term signature of the contents of a given
+tag.
 
 EXAMPLE CGITRC FILE
 -------------------