add comment with vim autoload for pism

proofs/simple.pism

@@ -15,8 +15,8 @@ start
     # Compute P = xG + A
     witness ak param:ak
     assert_not_small_order ak
-
     fr_as_binary_le secret param:secret
+
     ec_mul_const public secret G_SPEND
 
     ec_add public public ak

scripts/pism.vim

@@ -1,3 +1,13 @@
+"For autoload, add this to your VIM config:
+"   VIM:    .vimrc
+"   NeoVIM: .config/nvim/init.vim
+"
+"autocmd BufRead *.pism call SetPismOptions()
+"function SetPismOptions()
+"    set syntax=pism
+"    source /home/narodnik/src/sapvi/scripts/pism.vim
+"endfunction
+
 if exists('b:current_syntax')
     finish
 endif