docs: clarify libvterm repo isn't used anymore

Author: dundargoc

README.md

@@ -1,4 +1,9 @@
 # libvterm fork
 
+**Important**: This is currently not used by neovim. Vterm has instead been
+bundled into the neovim repo itself:
+https://github.com/neovim/neovim/tree/master/src/nvim/vterm. All pull requests
+should be sent to neovim instead.
+
 This is a fork of https://www.leonerd.org.uk/code/libvterm. An exact mirror
 of upstream is in the branch `mirror`.