i18n: move the scripts to the top-level 'i18n' directory

Also add a README.md to python3-flightgear explaining how to use the
PYTHONPATH environment variable or a .pth file in order to run the
Python scripts in FGMeta, and pointing to the top-level directories
'catalog' and 'i18n'.

python3-flightgear/README.md

@@ -0,0 +1,48 @@
+Python code for FlightGear “meta” work
+======================================
+
+The `flightgear` directory contains FlightGear-specific Python 3 modules.
+These modules are mostly of interest to FlightGear developers.
+
+
+Telling your Python interpreter how to access the modules
+---------------------------------------------------------
+
+In order to run most of the Python scripts in FGMeta, your Python 3
+installation must have the `/path/to/fgmeta/python3-flightgear` directory in
+its `sys.path`. One way to do this is to use something like the following in
+your shell setup:
+
+    export PYTHONPATH="/path/to/fgmeta/python3-flightgear"
+
+This example uses Bourne-style syntax; adjust for your particular shell.
+Several directories may be added this way using a colon separator on Unix, and
+presumably a semicolon on Windows.
+
+An alternative to setting `PYTHONPATH` is to add .pth files in special
+directories of your Python installation(s). For instance, you can create a
+file, say, `FlightGear-FGMeta.pth`, containing a single line (with no space at
+the beginning):
+
+    /path/to/fgmeta/python3-flightgear
+
+If you want the modules present in `/path/to/fgmeta/python3-flightgear` to be
+accessible to a particular Python interpreter (say, a Python 3.8), simply put
+the `.pth` file in `/path/to/python-install-dir/lib/python3.8/site-packages/`.
+This can even be a virtual environment if you want. For the system Python
+interpreters on Debian, you can put the `.pth` file in, e.g,
+`/usr/local/lib/python3.8/dist-packages/`. Note that you may add more lines to
+a `.pth` file in case you want to add other paths to the Python interpreter's
+`sys.path`.
+
+
+The scripts
+-----------
+
+Once you've done the above setup, the Python 3 scripts in FGMeta should run
+fine. This concerns in particular scripts located in the following top-level
+directories of FGMeta:
+
+    catalog   Generation of aircraft catalogs
+    i18n      Management of translations in FlightGear (i18n stands for
+              “internationalization”)