add sync-missing-metadata command with docs, resolves #25

docs/create-missing-metadata.md

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+# TubeSync
+
+## Advanced usage guide - creating missing metadata
+
+This is a new feature in v0.9 of TubeSync and later. It allows you to create or
+re-create missing metadata in your TubeSync download directories for missing `nfo`
+files and thumbnails.
+
+If you add a source with "write NFO files" or "copy thumbnails" disabled, download
+some media and then update the source to write NFO files or copy thumbnails then
+TubeSync will not automatically retroactively attempt to copy or create your missing
+metadata files. You can use a special one-off command to manually write missing
+metadata files to the correct locations.
+
+## Requirements
+
+You have added a source without metadata writing enabled, downloaded some media, then
+updated the source to enable metadata writing.
+
+## Steps
+
+### 1. Run the batch metadata sync command
+
+Execute the following Django command:
+
+`./manage.py sync-missing-metadata`
+
+When deploying TubeSync inside a container, you can execute this with:
+
+`docker exec -ti tubesync python3 /app/manage.py sync-missing-metadata`
+
+This command will log what its doing to the terminal when you run it.
+
+Internally, this command loops over all your sources which have been saved with
+"write NFO files" or "copy thumbnails" enabled. Then, loops over all media saved to
+that source and confirms that the appropriate thumbnail files have been copied over and
+the NFO file has been written if enabled.