docs: describe what the shell variables are for

I've had feedback that the variables are confusing as you may be
expecting to replace them if you've seen other install guides/etc. in
the past. Let's be clear about what they are to avoid confusion

Author: Minion3665

README.md

@@ -52,6 +52,21 @@ generated by this command is usable wherevere the language server is supported!
 
 # Copy/pasteable commands
 
+> [!NOTE]
+> In the following instructions we use some variables (`$AUTOGEN_FLAGS`,
+> `$CONFIGURE_FLAGS`, etc.). These **don't need to be replaced by you**.
+> Instead, the shell you are in will have them automatically populated.
+> 
+> Some of the flags are important, for example they might have paths
+> to dependencies, so you should keep them intact. For adding your own
+> customizations you can add additional flags to the command line. For example,
+> to set an external tarball directory you could run this in place of the
+> existing `./autogen.sh` line:
+>
+> ```
+> ./autogen.sh $AUTOGEN_FLAGS --with-external-tar=../../core-tarballs
+> ```
+
 Before you get started, you'll need to clone this repository to your system. We
 recommend that you keep it alongside your LibreOffice and Collabora Online code
 rather than putting one inside the other