docs: Add dragonfly BSD to OS reference table
@@ -58,6 +58,7 @@ These are provided by the `.system()` method call.
| cygwin | The Cygwin environment for Windows |
| haiku | |
| freebsd | FreeBSD and it's derivatives |
+| dragonfly | DragonFly BSD |
Any string not listed above is not guaranteed to remain stable in
future releases.
