[gimp] devel-docs: adding some links on how to find implementation details...

[Jehan Pagès <jehanp src gnome org>]

commit c48cbd1c1651e66a9d27ec4b4287604817e215c4
Author: Jehan <jehan girinstud io>
Date:   Fri May 18 18:52:04 2018 +0200

    devel-docs: adding some links on how to find implementation details...
    
    ... for layer modes.
    KDE developers asked me where the code was so that they could reproduce
    actual algorithms in their XCF reader. This is obviously interesting
    information to have around in our docs.

 devel-docs/xcf.txt |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)
---
diff --git a/devel-docs/xcf.txt b/devel-docs/xcf.txt
index 6138724..ae22ec9 100644
--- a/devel-docs/xcf.txt
+++ b/devel-docs/xcf.txt
@@ -1230,6 +1230,15 @@ PROP_MODE (essential)
   none) of the canvas, it will be the only layer to have its mode
   forced to Normal.
 
+  Implementation note: all layer modes are implemented as GEGL
+  operations. The list can be found at:
+    app/operations/layer-modes/gimp-layer-modes.c
+  The "op_name" value in particular gives the operation name allowing
+  reader developers to search for this string. For instance, the
+  "Normal" layer mode is implemented as the "gimp:normal" GEGL operation
+  whose implementation can be found at:
+    app/operations/layer-modes/gimpoperationnormal.c
+
 NOTE: The layer modes 'Old broken Overlay' and 'Soft light (legacy)' are identical.
 
 PROP_OFFSETS (essential)