docs: add missing dependency to build on alpine

[SethFalco]

Ran into the following while trying to build Siglo:

Program desktop-file-validate found: NO

--- stderr ---
Traceback (most recent call last):
  File "/home/user/Documents/repos/theironrobin/siglo/build-aux/meson/postinstall.py", line 16, in <module>
    call(['update-desktop-database', '-q', path.join(datadir, 'applications')])
  File "/usr/lib/python3.10/subprocess.py", line 345, in call
    with Popen(*popenargs, **kwargs) as p:
  File "/usr/lib/python3.10/subprocess.py", line 966, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/usr/lib/python3.10/subprocess.py", line 1842, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'update-desktop-database'

On Alpine the package is just called desktop-file-utils too. 👍🏾 Just adds it to the list of dependencies for building on Alpine.

---

While I was at it, I also made some subtle changes to the README.

• Made line breaks under headings more consistent.
• Replaced the empty heading at the bottom with an an actual horizontal line rather than kind of simulating one with the underline that headers get.
• Fixed a typo. (Bluetooth starts with an uppercase characters!)

[theironrobin]

thanks! :D