Additionally you can set special flags for Substitution by appending [flags] as the third argument to the RewriteRule directive. Flags is a comma-separated list of any of the following flags:
[...]
'qsappend|QSA' (query string append)
This flag forces the rewrite engine to append a query string part of the substitution string to the existing string, instead of replacing it. Use this when you want to add more data to the query string via a rewrite rule.