[skip ci] add upgrade note

Closes GH-5798

Author: remicollet

UPGRADING.INTERNALS

@@ -153,6 +153,10 @@ PHP 8.0 INTERNALS UPGRADE NOTES
         with extremely constrained memory, and compilation fails with an "out
         of memory" message, add "-O0" to CFLAGS.
 
+    5.  build system and provider are displayed in phpinfo from enviroment:
+        - PHP_BUILD_SYSTEM (default is same as PHP_UNAME)
+        - PHP_BUILD_PROVIDER (no default)
+
   b. Unix build system changes
 
     1.  --enable-maintainer-zts is renamed --enable-zts for parity with Windows