Update compatibility.md

per discussion here: f5ee642#commitcomment-79569628

Author: fredkilbourn

docs/compatibility.md

@@ -15,5 +15,6 @@ Below are the [semver](http://semver.org/) ranges representing which versions of
 * MongoDB Server 4.2.x: mongoose `^5.7.0` or `6.x`
 * MongoDB Server 4.4.x: mongoose `^5.10.0` or `6.x`
 * MongoDB Server 5.x: mongoose `^6.0.0`
+* MongoDB Server 6.x: mongoose `^6.5.0`
 
 Note that Mongoose 5.x dropped support for all versions of MongoDB before 3.0.0. If you need to use MongoDB 2.6 or older, use Mongoose 4.x.