[meld] README: Add note about Builder development (#311)

[Kai Willadsen <kaiw src gnome org>]

commit 777438dd6339b423e9d1d14d7b19d485a7e21b9b
Author: Kai Willadsen <kai willadsen gmail com>
Date:   Sat Apr 13 12:42:47 2019 +1000

    README: Add note about Builder development (#311)

 README.md | 12 ++++++++++++
 1 file changed, 12 insertions(+)
---
diff --git a/README.md b/README.md
index 51ccd4a8..2105dbdd 100644
--- a/README.md
+++ b/README.md
@@ -99,6 +99,18 @@ Windows installer can be built with command
 
 that creates file `dist/Meld-VERSION-ARCH.msi`
 
+
+Developing
+----------
+
+It's easy to get started developing Meld. From a git checkout, just run
+`bin/meld`.
+
+We also support development using Flatpak via GNOME Builder. At the Builder
+"Clone..." dialog, enter https://gitlab.gnome.org/GNOME/meld.git, and the
+default build + run development flow using Flatpak should work.
+
+
 Contributing
 ------------