[docs] Describe price info contribution

Author: dimkarakostas

docs/contribute.md

@@ -37,3 +37,16 @@ list of dictionaries with the following information:
 To contribute mapping information you can either update an existing file, by
 changing and/or adding some entries, or create a new file for a newly-supported
 ledger.
+
+### Price information
+
+The directory `price_data/` contains information about the supported ledgers'
+market price. Each file in this folder is named `<project_name>.csv` (for the
+corresponding ledger). The csv file has no header and each line contains two
+comma-separated values:
+(i) a day (in the form YYYY-MM-DD);
+(ii) the USD market price of the token on the set day.
+
+To contribute price information you can either update an existing file, by
+adding entries for days where data is missing, or create a new file for a
+newly-supported ledger and add historical price data.