Adjust README

README.md

@@ -479,7 +479,7 @@ Now, we only need to get the contents of that variable into the LaTeX source,
 and finally the compiled PDF.
 
 The LaTeX package `hyperref` can modify PDF metadata.
-In the LaTeX preamble, we can then use
+In the LaTeX preamble, we can then use, for example,
 
 ```latex
 \usepackage[pdfusetitle]{hyperref}% pdfusetitle reads from \author and \title
@@ -495,23 +495,22 @@ Note that in LaTeX, you likely used `\author{<author's name>}` and `\title{<docu
 somewhere in the preamble to generate a title page.
 `hyperref`'s `pdfusetitle` option will use those values for the PDF metadata.
 Lastly, `pdfcreator` will fill the `Application` field we see above.
-However, `\GitRefName{}` and `\GitShortSHA{}}` need to be defined.
-These are defined earlier in the preamble:
+
+However, `\GitRefName{}` and `\GitShortSHA{}}` need to be defined first.
+This happens earlier in the preamble:
 
 ```latex
-\newcommand*{\GitRefName}{n.a.}
-\newcommand*{\GitShortSHA}{n.a.}
+\directlua{
+ dofile("lib/lua/envvar_newcommands.lua")
+}
 ```
 
-(Side note: use the [starred variant of `newcommand`](https://tex.stackexchange.com/a/1057/120853),
-unless you need paragraph breaks, which is mostly not the case).
-
-Using the above definition, the fields will just show up as *not available* if for example
-compiling locally ("at home").
-The server has a script employing `sed` (since we're using GNU/Linux there),
-specified in [.gitlab-ci.yml](.gitlab-ci.yml), that seeks `GitRefName` and `GitShortSHA`
-and replaces whatever comes in the curly braces of those commands.
-See the `script` of the `replace_git_metadata` job in that file.
+This runs, with the help of LuaTeX's [Lua](https://www.lua.org/about.html) integration,
+some [Lua code](lib/lua/envvar_newcommands.lua) directly in LaTeX.
+Within Lua, `os.getenv(<string> name)` lets us retrieve environment variables, which are
+then turned into macros/control sequences (`\newcommand`) of a given name using
+`token.set_macro(<string> csname, <string> content)`, see the [LuaTeX reference](https://mirrors.ctan.org/systems/doc/luatex/luatex.pdf)
+on all the included Lua functionality provided by LuaTeX, on top of regular Lua.
 
 ### Add PDF Download Button