Documentation: explain how to add special Code128 codes to barcode

I am using JsBarcode to make a barcode scanner programming sheet for our application. Our barcode scanner adds some unusual Code128 symbols at the start to make sure that regular barcodes do not cause it to change its settings. For our scanner the programming codes are always in format: START B, FNC3, programming code, STOP.

Forcing Code128B is well documented, but inserting the FNC3 symbol (code 96) wasn't so clear from the documentation. After looking at the source code I noticed it was just a matter of adding 105 to the symbol code and encoding in the string parameter as hex. So FNC3 becomes 96+105 = 201 = C9 hex. For example to reset our barcode scanner to factory settings (START B, FNC3, "000B0", STOP) this generated the right programming barcode:

```
JsBarcode("#barcode_1","\xC9000B0",{format:"CODE128B"});
```

Start by locating the project's documentation and the source code for CODE128B handling. Document how to encode FNC3 using the hexadecimal value described in the issue, including the provided JsBarcode example. Done means a newcomer can find and follow the instructions for generating the scanner programming barcode.

Written by the indexing model from the issue text.