add software verison

Author: bigtreetech

Copy to SD Card root directory to update/README.md

@@ -1,4 +1,4 @@
-### how to update firmware
+# how to update firmware
 copy [these files](https://github.com/bigtreetech/BIGTREETECH-TouchScreenFirmware/tree/master/Copy%20to%20SD%20Card%20root%20directory%20to%20update) to SD Card root directory
 
 The structure of the files in SD Card should be as follows
@@ -8,3 +8,10 @@ The structure of the files in SD Card should be as follows
 - the **TFT24** folder contains the icons and fonts
   - icons: If you want to customize icons, see [here](https://github.com/bigtreetech/BIGTREETECH-TFT35-V1.2/blob/master/TFT%2035%20screen%20customize.pdf), and update the folder by SD Card (don't need update every time)
   - fonts: this is a bitmap fonts of ASCII and GBK. The font is Song typeface (don't need update every time)
+
+# software version
+### Vx.x.20
+- The first version of the Integration Project
+### Vx.x.21
+- fix unable to print properly from U disk(Stop printing at random)
+- fix busy popup with "Processing" in Printing and Stop Printing