[gdm: 4/11] README: Update build instructions using meson

[Ray Strode <halfline src gnome org>]

commit 663612fca721e1ecdfd3c43cd3f484173fa14c80
Author: Marco Trevisan (Treviño) <mail 3v1n0 net>
Date:   Fri Aug 28 07:03:34 2020 +0200

    README: Update build instructions using meson

 README.md | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)
---
diff --git a/README.md b/README.md
index 38dd80eb..63737c16 100644
--- a/README.md
+++ b/README.md
@@ -9,10 +9,9 @@ providing graphical log-ins and managing local and remote displays.
 To build and install GDM from source, just execute the following commands:
 
 ```
-$ ./autogen.sh
-$ ./configure
-$ make
-$ sudo make install
+$ meson _build
+$ ninja -C _build
+$ sudo ninja -C _build install
 ```
 
 ## Contributing