add short legacy vimscript intro

Author: Konfekt

vim9script.md

@@ -12,7 +12,22 @@ It improves performance, readability, and structure over legacy Vimscript.
 Try [vim9-conversion-aid](https://github.com/ubaldot/vim9-conversion-aid) as a starting point to convert legacy Vimscript to Vim9Script.
 
 ```vim
+" Vim script (also called VimL) is the subset of Vim's ex-commands which
+" supplies a number of features one would expect from a scripting language,
+" such as values, variables, functions or loops. Always keep in the back of
+" your mind that a Vim script file is just a sequence of ex-commands. It is
+" very common for a script to mix programming-language features and raw
+" ex-commands.
+"
+" You can run Vim script directly by entering the commands in command-line mode
+" (press : to enter command-line mode), or you can write them to a file
+" (without the leading :) and source it in a running Vim instance (:source " path/to/file). Some files are sourced automatically as part of your
+" configuration (see |startup|). This guide assumes that you are familiar
+" with ex-commands and will only cover the scripting. Help topics to the
+" relevant manual sections are included.
+
 vim9script
+
 # The vim9script namespace, above, is required to distinguish a Vim9 script
 # *.vim file from a legacy vimscript file.  In Vim's command-line mode,
 # or in a legacy script, using the command `:vim9cmd` (or just `:vim9`) before