Document some more LaTeX idiosyncracies

README.md

@@ -82,6 +82,40 @@ plugins.
 Mostly you should just use standard LaTeX in OTS Doctools documents.
 However, there are some things you should know:
 
+### You can set jinja variables at the top of your document.
+
+You can put jinja variables in a special block at the very top of your
+input file:
+
+```
+  ---
+  title: "My Life as an Eggplant Hunter:\\\\Nobody Told Me They Could Run So Fast"
+  date: 31 Nov 2023
+  draft: true
+  fish: halibut
+  ---
+```
+
+Certain variables will be treated specially.  For example, the values
+of `title` and `date` will be displayed on the title page.  (Remember,
+backslashes are doubled here because they need to be quoted for
+jinja.)  If the value of `draft` is `true`, then every page will get a
+large, light gray "DRAFT" diagonal watermark.
+
+Other variables are just there to be referred to later.  For example,
+in the above example, the value of `fish` is `halibut`, so if you put
+this in your latex:
+
+```
+  I always ate eggplant, even when offered \VAR{fish}.
+```
+
+It will render as this in the output:
+
+```
+  I always ate eggplant, even when offered halibut.
+```
+
 ### Avoid accidentally triggering jinja
 
 If you start a LaTeX comment using two or more percent signs together