[gnome-disk-utility] Build/install readme: Give concrete instructions for libblockdev development

[Kai Lüke <kailueke src gnome org>]

commit 9dc670d39048e1ca06e6f64494f0b7c03e4fdb48
Author: Kai Lüke <kailueke riseup net>
Date:   Tue Aug 11 22:59:24 2020 +0200

    Build/install readme: Give concrete instructions for libblockdev development

 INSTALL | 15 +++++++++++++++
 1 file changed, 15 insertions(+)
---
diff --git a/INSTALL b/INSTALL
index 849dcc1d..78d2eddf 100644
--- a/INSTALL
+++ b/INSTALL
@@ -90,3 +90,18 @@ Build a Flatpak app with your local source code changes:
 
 If you want to test things in your own development branches of UDisks or libblockdev,
 you need to change the Flatpak manifest and point it to your repository/branch.
+Here an example for the temporary change you need to do for libblockdev:
+diff --git a/flatpak/org.gnome.DiskUtility.json b/flatpak/org.gnome.DiskUtility.json
+index 2f89e2e3..16a20deb 100644
+--- a/flatpak/org.gnome.DiskUtility.json
++++ b/flatpak/org.gnome.DiskUtility.json
+@@ -189,7 +189,8 @@
+             "sources" : [
+                 {
+                     "type" : "git",
+-                    "url" : "https://github.com/storaged-project/libblockdev.git";
++                    "url" : "https://github.com/MYFORK/libblockdev.git";,
++                    "branch" : "MYBRANCH"
+                 }
+             ]
+         },