Updated documentation.

Author: Jim Norton

docs/Signing.md

@@ -196,6 +196,12 @@ Provides a value to be set with a custom tag
    Value argument is in the form of a hex string, e.g. `--custom-tlv-buffer 0x0030 AABBCCDDEE`
    will add a TLV entry with tag 0x0030, length 5 and value 0xAABBCCDDEE.
 
+   * `--custom-tlv-string tag ascii-string`: Adds a TLV entry with arbitrary length to the manifest
+   header, corresponding to the type identified by `tag`, and assigns the value of `ascii-string`. The
+   tag is a 16-bit number. Valid tags are in the range between 0x0030 and 0xFEFE. The length
+   is implicit, and is the length of the `ascii-string`. `ascii-string` argument is in the form of a string,
+   e.g. `--custom-tlv-string 0x0030 "Version-1"` will add a TLV entry with tag 0x0030,
+   length 9 and value Version-1.
 
 #### Three-steps signing using external provisioning tools