update readme

README.md

@@ -98,6 +98,7 @@ makeself.sh [args] archive_dir file_name label startup_script [script_args]
     * **`--follow`** : Follow the symbolic links inside of the archive directory, i.e. store the files that are being pointed to instead of the links themselves.
     * **`--append`** _(new in 2.1.x)_: Append data to an existing archive, instead of creating a new one. In this mode, the settings from the original archive are reused (compression type, label, embedded script), and thus don't need to be specified again on the command line.
     * **`--header`** : Makeself uses a separate file to store the header stub, called `makeself-header.sh`. By default, it is assumed that it is stored in the same location as makeself.sh. This option can be used to specify its actual location if it is stored someplace else.
+    * **`--preextract`** : Specify a pre-extraction script. The script is executed with the same environment and initial `script_args` as `startup_script`.
     * **`--cleanup`** : Specify a script that is run when execution is interrupted or finishes successfully. The script is executed with the same environment and initial `script_args` as `startup_script`. 
     * **`--copy`** : Upon extraction, the archive will first extract itself to a temporary directory. The main application of this is to allow self-contained installers stored in a Makeself archive on a CD, when the installer program will later need to unmount the CD and allow a new one to be inserted. This prevents "Filesystem busy" errors for installers that span multiple CDs.
     * **`--nox11`** : Disable the automatic spawning of a new terminal in X11.