Document register --sort in Register.md

hledger/Hledger/Cli/Commands/Register.md

@@ -16,6 +16,8 @@ Flags:
                             description closest to DESC
   -r --related              show postings' siblings instead
      --invert               display all amounts with reversed sign
+     --sort=FIELDS          comma-separated list of fields used to sort
+                            transactions
   -w --width=N              set output width (default: terminal width or
                             $COLUMNS). -wN,M sets description width as well.
      --align-all            guarantee alignment across all lines (slower)
@@ -80,6 +82,13 @@ displayed as negative numbers.
 It's also useful to show postings on the checking account together with the
 related account:
 
+The `--sort=FIELDS` flag sorts by the fields given, which can be any of 
+`account`, `amount`, or `date`, optionally separated by commas.
+For example, `--sort account,amount` will group all transactions in each
+account, sorted by transaction amount.
+Each field can be negated by a preceding `-`, so `--sort -amount` will show
+transactions ordered from smallest amount to largest amount.
+
 ```cli
 $ hledger register --related --invert assets:checking
 ```