[gimp] INSTALL: update info about libmypaint.

[Jehan <jehanp src gnome org>]

commit 1028345b7b298ebb9be1af7d65b94d9b60ed9294
Author: Jehan <jehan girinstud io>
Date:   Mon Apr 1 17:04:56 2019 +0200

    INSTALL: update info about libmypaint.
    
    Since recently, one can also install the new "libmypaint-v1" branch. It
    is not so different, but has several fixes. Among them, it fixes
    building with recent automake.

 INSTALL.in | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/INSTALL.in b/INSTALL.in
index de7aadbd12..109f03c1f7 100644
--- a/INSTALL.in
+++ b/INSTALL.in
@@ -85,6 +85,9 @@ header files installed.
      If installing from repository, do not install the master branch!
      Checkout the tag "v1.3.0" instead, or simply install from a tarball
      or from your favorite package manager.
+     You may also checkout the "libmypaint-v1" branch, which is the
+     development branch for libmypaint v1.x and has some more recent
+     fixes.
 
  11. We also need the mypaint-brushes data package: