docs: update readme and example env with withdrawal info

Author: thephez

.env.example

@@ -1,5 +1,8 @@
 MNEMONIC='jewel pattern cry forget gown better agent celery nothing glove silk ill'
 # Mnemonic's first address: ydinxECoSdvGkCMZFhrqY4fg3Jc3AbeS2z
+# WITHDRAWAL_ADDRESS is an Core chain address from Dash Core or another wallet
+#  Credits withdrawn from Platform will be received as DASH in the address
+WITHDRAWAL_ADDRESS='ydinxECoSdvGkCMZFhrqY4fg3Jc3AbeS2z'
 
 # Core block height to begin syncing from
 #  Allows syncing to complete faster than if syncing the entire chain from the first block

README.md

@@ -34,6 +34,9 @@ npm ci
    the address reported in the previous step
 1. Open the `.env` file (See [`.env.example`](./.env.example) for an example
    `.env` file) and set `MNEMONIC` to the wallet mnemonic from step 3.
+1. If doing withdrawals, open your `.env` file and set `WITHDRAWAL_ADDRESS` to
+   the Core chain address where you would like to receive funds withdrawn from
+   Platform.
 1. (Optional) To minimize wallet sync time, open your `.env` file and set
    `SYNC_START_HEIGHT` to a Core chain block height just below the height of
    your wallet's first transaction. Otherwise you can skip this step and use the