docs: update README

Author: uerceg

README.md

@@ -902,6 +902,7 @@ The callback function will be called after the SDK receives the final attributio
 - `costType` the cost type string
 - `costAmount` the cost amount
 - `costCurrency` the cost currency string
+- `fbInstallReferrer` the Facebook install referrer information
 
 **Note**: The cost data - `costType`, `costAmount` & `costCurrency` are only available when configured in `AdjustConfig` by setting `needsCost` member to `true`. If not configured or configured, but not being part of the attribution, these fields will have value `null`. This feature is available in SDK v4.26.0 and later.