add a VERSIONS note about SLiMv6MutationModel

Author: bhaller

VERSIONS

@@ -214,6 +214,7 @@ multitrait branch:
 		the schema for the mutation table metadata now encodes an array (of length determined by the metadata itself) of int64_t SLiM mutation IDs
 	fix #666, remove `flags` from individual metadata (shifting the migrant flag, now named SLIM_TSK_INDIVIDUAL_MIGRATED, over to the flags column of the individual table)
 		note this entails changes to the individual table flags column, the individual table metadata and schema, and SLiM's provenance entries (to add a description of the flag)
+	SLiMMutationModel has become SLiMv6MutationModel; this is not a change in SLiM, but it should be noted in the SLiM 6 release notes, with a link to pyslim docs showing how to migrate over
 
 
 version 5.2 (Eidos version 4.2):