docs: add readme of the MFB MVB Appender component

Author: danielkondys

doc/source/mfb.rst

@@ -11,6 +11,7 @@ Components using the MFB bus are typically located in the ``comp/mfb_tools/`` di
 
    comp/mfb_tools/debug/gen_loop_switch/readme
    comp/mfb_tools/debug/generator/readme
+   comp/mfb_tools/edit/frame_appender/readme
    comp/mfb_tools/edit/frame_extender/readme
    comp/mfb_tools/edit/frame_trimmer/readme
    comp/mfb_tools/flow/crossbarx_stream2/readme