sdl12-compat: README.md: Note that environment variables are not to be modified.

From d764cc68f81228ccb6d864b7bc7d0a4ce66bfccd Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Thu, 1 Sep 2022 19:12:15 -0400
Subject: [PATCH] README.md: Note that environment variables are not to be
 modified.

Fixes #188.
---
 README.md | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/README.md b/README.md
index f6f489fb..2c120bf9 100644
--- a/README.md
+++ b/README.md
@@ -144,6 +144,13 @@ SDL12COMPAT_HIGHDPI=1 SDL12COMPAT_OPENGL_SCALING=0 %command%
 will run `%command%` with high-dpi monitor support enabled, but OpenGL
 scaling support disabled.
 
+(While these environment variables are checked at various times throughout
+the lifetime of the app, sdl12-compat expects these to be set before the
+process starts and not change during the life of the process, and any
+places where changing it later might affect operation is purely accidental
+and might change. That is to say: don't write an SDL 1.2-based app with
+plans to tweak these values on the fly!)
+
 The available options are:
 
 - SDL12COMPAT_DEBUG_LOGGING: