[gnome-network-displays] README: Add build deps install prerequisite

[Benjamin Berg <bberg src gnome org>]

commit 52e98002b8f03cb045d4240e13bf4b341af2a21c
Author: Christian Glombek <lorbus fedoraproject org>
Date:   Tue Jan 25 11:42:51 2022 +0100

    README: Add build deps install prerequisite
    
    Closes: !150

 README.md | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/README.md b/README.md
index 8f37f34..3c076b5 100644
--- a/README.md
+++ b/README.md
@@ -18,6 +18,7 @@ Build
 To build it locally:
 
 - clone the repository
+- install build dependencies (e.g. on Fedora with `dnf build-dep gnome-network-displays`)
 - install `meson`
 - run `meson build` on the cloned repository
 - run `meson install` on the `build` folder created by meson