Add instructions for batch file updates

The key is the recently-added flag `--no-top-level-comment` or `--no-tlc` which
avoids adding the "TODO" comments to many files, thus not requiring manual
cleanup afterwards.

Merge pull request mbrukman#11 from mbrukman/test-file-rename

Rename `*_tests.sh` to `*_test.sh`.

Added support for *.hh as C++ files.

Also added a TODO for handling *.h files optionally as C++ files rather than C
files, with accordingly different comment characters.