[gimp] INSTALL: update with libheif required dependency for AVIF support.

[Jehan <jehanp src gnome org>]

commit 987325f1a7edc3c6f0e9fc1faa9d08707c9e8f3a
Author: Jehan <jehan girinstud io>
Date:   Tue Sep 1 18:38:46 2020 +0200

    INSTALL: update with libheif required dependency for AVIF support.

 INSTALL.in | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/INSTALL.in b/INSTALL.in
index 10e5a32faf..7a0d1e7fee 100644
--- a/INSTALL.in
+++ b/INSTALL.in
@@ -114,8 +114,9 @@ header files installed.
         https://github.com/strukturag/libheif
 
      Make sure you build libheif with libde265 and libx265 support (for
-     respectively decoding and encoding), otherwise the plug-in is
-     mostly useless.
+     respectively decoding and encoding of HEVC, i.e. HEIC files), and
+     libaom decoder and encoder (for AV1, i.e. AVIF files), otherwise
+     the plug-in is mostly useless.
 
  14. GObject Introspection requires the following dependencies to be
      built and installed with introspection as well: cairo, GdkPixbuf,