docs: Add freebsd to the reference table
@@ -57,6 +57,7 @@ These are provided by the `.system()` method call.
| windows | Any version of Windows |
| cygwin | The Cygwin environment for Windows |
| haiku | |
+| freebsd | FreeBSD and it's derivatives |
Any string not listed above is not guaranteed to remain stable in
future releases.
