schemas: commodity: Initial README

schemas/commodity-README.md

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+# EDDN BlackMarket Schema
+
+## Introduction
+Here we document how to take data from an ED `Market` Journal Event and
+properly structure it for sending to EDDN.
+
+Please consult [EDDN Schemas README](./README-EDDN-schemas.md) for general
+documentation for a schema such as this.
+
+## Senders
+The primary data source for this schema is the ED Journal event `Market`, 
+and the additional file, `Market.json`, that it signals the writing of.
+
+So, look for the `Market` event, and when it occurs open and read the 
+`Market.json` file which contains the actual data.  Treat *that* data as 
+the event.
+
+It *is* also possible to construct this data from a CAPI `/market` query.
+
+### Key Renames
+Many of the key names have a different case defined in this schema, make 
+sure you are renaming them as appropriate.
+
+#### StarSystem to systemName
+Rename the `StarSystem` key name to `systemName`.
+
+### Elisions
+#### Remove _Localised key/values
+You MUST remove the following key/value pairs from the data:
+
+  - `StationType` key/value.
+  - All keys whose name ends with `_Localised`, i.e. the `Name_Localised` 
+    key/values in Items.
+  - `Producer` key/value pair in Items.
+  - `Rare` key/value pair in Items.
+  - `id` key/value pair in Items.
+
+#### Item Category
+Remove not only the `Category_Localised` key/value, as above, but also the
+`Category` key/value pair from each Item.
+
+### Augmentations
+#### horizons flag
+Use the value from the `LoadGame` event.
+#### odyssey flag
+Use the value from the `LoadGame` event.