Add info to build.h on how to untrack changes

code/espurna/config/build.h

@@ -1,3 +1,5 @@
 // DO NOT EDIT THIS FILE MANUALLY
 // This file is modified by PlatformIO
+// This file should not be pushed when modified, untrack changes with:
+//  git update-index --assume-unchanged code/espurna/config/build.h
 #define APP_BUILD_FLAGS ""