[gimp] NEWS: add some "Known issues" section to 2.99.2 notes.

[Jehan <jehanp src gnome org>]

commit 94747df92ec9ebdbe900c2e3a314c914fa44d8fe
Author: Jehan <jehan girinstud io>
Date:   Thu Jun 4 22:32:02 2020 +0200

    NEWS: add some "Known issues" section to 2.99.2 notes.
    
    I feel that such section would matter particularly to show some major
    issues which we want to finish before a stable release.
    This is not necessarily an exhaustive list of blockers, just some major
    items.

 NEWS | 13 +++++++++++++
 1 file changed, 13 insertions(+)
---
diff --git a/NEWS b/NEWS
index 5e752ea410..79fcb07aec 100644
--- a/NEWS
+++ b/NEWS
@@ -101,3 +101,16 @@ Build:
     - a meson cross-build with Mingw-w64 for Windows 32-bit
     - a meson cross-build with Mingw-w64 for Windows 64-bit
     - static code analysis with cppcheck
+
+Known blocker issues:
+
+  - GTK+3 port is functional but not complete as some deprecated APIs
+    are still being used.
+  - Space invasion is still a work-in-progress.
+  - Plug-ins API update is still a work-in-progress.
+  - GIMP extensions are still a work-in-progress.
+  - Multiple layer selection is still work-in-progress. Some issues are
+    expected, and possibly even some crashes in code paths which were
+    not updated yet.
+  - No custom theme yet, in particular we want a neutral gray theme with
+    light/dark variants, and likely a middle-gray theme too.