Add @ prefix note for codemeta2cff

To help user avoid/workaround issue like one in
#26

Signed-off-by: Arthit Suriyawongkul <arthit@gmail.com>

Author: bact

codemeta2cff.1.md

@@ -16,6 +16,9 @@ Reads codemeta.json file and writes CITATION.cff. By default
 it assume both are in the current directory.  You can also
 provide the name and path to both files.
 
+Note that the 'id' and 'type' fields in codemeta.json
+require an '@' prefix.
+
 # OPTIONS
 
 -help