Add comments to our generated .gitignore, including a note about when to use a global gitignore. Remove .sass-cache since we use /tmp/sass-cache, which is already ignored.

railties/lib/rails/generators/rails/app/templates/gitignore

@@ -1,5 +1,15 @@
-.bundle
-db/*.sqlite3
-log/*.log
-tmp/
-.sass-cache/
+# See http://help.github.com/ignore-files/ for more about ignoring files.
+#
+# If you find yourself ignoring temporary files generated by your text editor
+# or operating system, you probably want to add a global ignore instead:
+#   git config --global core.excludesfile ~/.gitignore_global
+
+# Ignore bundler config
+/.bundle
+
+# Ignore the default SQLite database.
+/db/*.sqlite3
+
+# Ignore all logfiles and tempfiles.
+/log/*.log
+/tmp