Add installation instructions mentioning git submodules

[tfardet]

I'm not sure what's the best place to add this but there really should be a prominent warning about the fact that this repo uses git submodules and how to initialize/update them (I would personally add it to both README and docs, probably in an "installing" or "contributing" section, but that's just me) .

Problem:

At the moment, the documentation to add a region states that one can test the added feeds locally via /src/fetch.py feeds/<region>.json.

Installing gtfstidy is already a bit challenging due to patrickbr/gtfstidy#25 (I ended up using @magnusburton's fork with go install github.com/magnusburton/gtfstidy@latest instead).
However, even with gtfstidy installed, the command leads to

Traceback (most recent call last):
  File "/home/tfardet/Documents/Code/transitous/./src/fetch.py", line 188, in <module>
    fetcher = Fetcher()
              ^^^^^^^^^
  File "/home/tfardet/Documents/Code/transitous/./src/fetch.py", line 39, in __init__
    self.transitland_atlas = transitland.Atlas.load(
                             ^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/tfardet/Documents/Code/transitous/src/transitland.py", line 21, in load
    for f in Path(path / "feeds").iterdir():
  File "/usr/lib/python3.11/pathlib.py", line 931, in iterdir
    for name in os.listdir(self):
                ^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: 'transitland-atlas/feeds'

people not familiar with submodules will just not understand what's happening.

After I started looking for it, I found a mention here, but it's listed in "running a local instance", and that's likely not what most people will do: they'll just want to contribute some data and check/debug locally.

[jbruechert]

Should be fixed with 2228f08