Hint at not contributing user-specific Git ignores

Author: sschuberth

.gitignore

@@ -18,3 +18,6 @@ $/tests/fixtures/commit-graphs/
 
 # newer Git sees these as precious, older Git falls through to the pattern above
 $**/fuzz/Cargo.lock
+
+# Instead of adding more environment-specific ignores here, like for the IDE in use, prefer Git's user-global
+# `core.excludesFile` mechanism, see https://git-scm.com/docs/git-config#Documentation/git-config.txt-coreexcludesFile.