add undocumented "ms" rotation period to docs

README.md

@@ -709,7 +709,7 @@ files, or for rotating when the log reaches a threshold size.
 <td>No</td>
 <td>1d</td>
 <td>The period at which to rotate. This is a string of the format
-"$number$scope" where "$scope" is one of "h" (hours), "d" (days), "w" (weeks),
+"$number$scope" where "$scope" is one of "ms" (milliseconds), "h" (hours), "d" (days), "w" (weeks),
 "m" (months), "y" (years). Or one of the following names can be used
 "hourly" (means 1h), "daily" (1d), "weekly" (1w), "monthly" (1m),
 "yearly" (1y). Rotation is done at the start of the scope: top of the hour (h),