* docs: initially commit WS_TRADE_UPDATES.md
* docs/WS_TRADE_UPDATES: shorter text
* docs: rename WS_TRADE_UPDATES to WS_ORDER_EVENTS as that is more descriptive of what it documents
* docs/ADD_NEW_EXCHANGE: reference the newly added WS_ORDER_EVENTS file
* WS_ORDER_EVENTS: common terminology
* WS_ORDER_EVENTS: turn the code segment into a markdown table
* WS_ORDER_EVENTS: AverageExecutedPrice is now not mandatory
* exchanges/README: link to WS_ORDER_EVENTS