readme: update VSFilterMod status

Since active'ish forks focus on internal use, they might not appreciate
getting potential external users prominently redirected their way, so
do not link them atm. Should they expand scope we can add links later.

README.md

@@ -46,8 +46,9 @@ Other ASS/SSA implementations:
   - [xy-VSFilter/XySubFilter](https://github.com/Cyberbeing/xy-VSFilter/)
     - [pfmod](https://github.com/pinterf/xy-VSFilter/)
   - VSFilter in [MPC-HC](https://github.com/clsid2/mpc-hc/tree/develop/src/filters/transform/VSFilter/)
-  - [VSFilterMod](https://code.google.com/archive/p/vsfiltermod/) (with custom format extensions)
-    - [sorayuki fork](https://github.com/sorayuki/VSFilterMod/) with some bugfixes
+  - [VSFilterMod](https://code.google.com/archive/p/vsfiltermod/) with custom format extensions (defunct, subsumed by forks)
+    - [sorayuki fork](https://github.com/sorayuki/VSFilterMod/) with some bugfixes (defunct)
+    - various forks focussing on internal use
   - [Threaded VSFilter](https://code.google.com/p/threaded-vsfilter/) (defunct)
   - VSFilter in [Guliverkli2](http://sourceforge.net/projects/guliverkli2/) (defunct, subsumed by all of the above)
   - VSFilter in [guliverkli](http://sourceforge.net/projects/guliverkli/) (defunct, forked as Guliverkli2)