chore: add skip existing to import command (#256)

Signed-off-by: Mark Phelps <[email protected]>

cli/commands/import.mdx

@@ -14,6 +14,7 @@ flipt import [flags] <file>
       --config string    path to config file
       --drop             drop database before import
   -h, --help             help for import
+      --skip-existing    only import new data
       --stdin            import from STDIN
   -t, --token string     client token used to authenticate access to remote Flipt instance when importing.
 ```