Document direct header inclusion valid since 2.6.1.

Since commit e13391333 'Make macros for header file names optional.' the
header file layout has become public and using the macro expansions for
includes is no longer required.  While this happened in 2.10.3, the header
file layout had not changed since commit 392cf22f8a5 'Another adjustment to
header locations.' in 2.6.1.  Retroactively make the header file layout
public back to 2.6.1 to speed adoption.
diff --git a/docs/INSTALL.ANY b/docs/INSTALL.ANY
index f36c2e0..3136613 100644
--- a/docs/INSTALL.ANY
+++ b/docs/INSTALL.ANY
@@ -10,7 +10,7 @@
 
   * If you use macro names  for FreeType header files (while mandatory
     in earlier versions,  this is now optional  since FreeType version
-    2.10.3) it  is necessary to disable pre-compiled headers.  This is
+    2.6.1) it  is necessary to  disable pre-compiled headers.  This is
     very important for Visual C++, because lines like
 
       #include FT_FREETYPE_H