You can generate xxhsum.1 by the following procedure:
```
sudo apt-get install -y ruby-ronn
# make man generates xxhsum.1 from xxhsum.1.md #
make man
# edit xxhsum.1.md here #
# auto preview for development
make preview-man
```
`Makefile` has the following tricky part:
```
xxhsum.1: xxhsum.1.md
cat $^ | $(MD2ROFF) $(MD2ROFF_FLAGS) | sed -n '/^\.\\\".*/!p' > $@
```
Last `sed` command removes `ronn`'s comment lines (`ronn`'s version, author, URL).
It doesn't mean disrespect for `ronn`. We need this removal because we must
ignore `ronn`'s version to minimize diff.
