Document pandoc requirement

[goneall]

It looks like a recent change to the spec parser introduced a dependency on pandoc.

This should be documented in the README.

Running spec-parser without pandoc installed results in the following error:

File "C:\Users\gary\git\spec-parser\spec_parser\tex.py", line 130, in markdown_to_tex
    process = subprocess.run(
              ^^^^^^^^^^^^^^^
  File "C:\Users\gary\AppData\Local\Programs\Python\Python311\Lib\subprocess.py", line 548, in run
    with Popen(*popenargs, **kwargs) as process:
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\gary\AppData\Local\Programs\Python\Python311\Lib\subprocess.py", line 1026, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "C:\Users\gary\AppData\Local\Programs\Python\Python311\Lib\subprocess.py", line 1538, in _execute_child
    hp, ht, pid, tid = _winapi.CreateProcess(executable, args,
                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
FileNotFoundError: [WinError 2] The system cannot find the file specified

[zvr]

It will also be in the code, since the dependency is only when generating TeX output.