Catalogs: Addition of an initial README.md file.

catalog/README.md

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+Introduction
+============
+
+This is the directory containing the script for updating aircraft, or other
+craft, catalogs.  It will create the `catalog.xml` file used to add a hangar to
+FlightGear, as well as the zip archives of each craft in the hangar and the
+md5sums, thumbnails, and previews of these.  It consists of the script:
+
+* update-catalog.py
+
+And its Python modules:
+
+* catalog.py
+* catalogTags.py
+* sgprogs.py
+
+
+Usage
+=====
+
+The script can be run directly from this directory, or the script and its
+modules can be copied together and run from any location.  The steps to use
+these are:
+
+* Create an output directory where the catalog and zip files will be located.
+* Copy the configuration files `catalog.config.xml`, `template.xml`, and
+  `zip-excludes.lst` from one of the `*catalog*` example directories into the
+  output directory.
+* Modify these files as required.
+
+Then run the script with:
+
+`$ $FGMETA/catalog/update-catalog.py dir`
+
+where `dir` is the output directory.  The script will create the following
+files:
+
+* `md5sum.xml`:  A file containing checksums of all craft zip archives in the
+  base output directory.
+* `ftp/catalog.xml`:  The XML catalog to upload to a server and advertise to
+  FlightGear users.
+* `ftp/*.zip`:  The zip archives of each craft in the hangar.
+* `ftp/previews/*_Splashs/`:  A directory per craft containing the splash screen
+  graphics.
+* `ftp/thumbnails/`:  The collection of thumbnail graphics for the hangar.
+
+The `ftp` directory is to be uploaded to a server which can be configured via
+the `catalog.config.xml` file.
+
+
+Examples
+========
+
+A number of example configuration files are located in this directory.  These
+include:
+
+* `fgaddon-catalog/`:  The configuration files used for the [official FGAddon
+  hangar](http://wiki.flightgear.org/FGAddon).
+* `stable-2018-catalog/':  The configuration files used for the 2018 long term
+  stability release.
+* `single-craft-catalog-test/`:  Configuration files used for testing the
+  catalog and zip archive creation for a single craft.  These are for content
+  developers to test their craft.