;doc: csv: add new coinbase example

hledger/hledger.m4.md

@@ -3757,6 +3757,31 @@ The balance assertions don't raise an error above, because we're
 reading directly from CSV, but they will be checked if these entries
 are imported into a journal file.
 
+### Coinbase
+
+A simple example with some CSV from Coinbase. The spot price is recorded using cost notation. 
+The legacy `amount` field name conveniently sets amount2 to the total cost.
+```csv
+# Timestamp,Transaction Type,Asset,Quantity Transacted,Spot Price Currency,Spot Price at Transaction,Subtotal,Total (inclusive of fees and/or spread),Fees and/or Spread,Notes
+# 2021-12-30T06:57:59Z,Receive,USDC,100,GBP,0.740000,"","","","Received 100.00 USDC from an external account"
+```
+```rules
+# coinbase.csv.rules
+skip 1
+fields Timestamp,Transaction_Type,Asset,Quantity_Transacted,Spot_Price_Currency,Spot_Price_at_Transaction,Subtotal,Total,Fees_Spread,Notes
+date %Timestamp
+date-format %Y-%m-%dT%T%Z
+description %Notes
+account1 assets:coinbase:cc
+amount %Quantity_Transacted %Asset @ %Spot_Price_at_Transaction %Spot_Price_Currency
+```
+```shell
+$ hledger print -f coinbase.csv
+2021-12-29 Received 100.00 USDC from an external account
+    assets:coinbase:cc    100 USDC @ 0.740000 GBP
+    income:unknown                 -74.000000 GBP
+```
+
 ### Amazon
 
 Here we convert amazon.com order history, and use an if block to