meson: use top-level directory for main meson.build file

It makes more sense to have a top-level meson.build file, for any
projects that want to use this as a subproject.

Signed-off-by: Steven Noonan <steven@valvesoftware.com>

README.md

@@ -29,7 +29,7 @@ But even if you don't make games or aren't on Steam, feel free to use this code
 This has only really been tested on Ubuntu 17.10.
 
 ```
-$ meson src build
+$ meson . build
 $ ninja -C build
 ```
 

meson.build

@@ -0,0 +1,7 @@
+project('GameNetworkingSockets', 'cpp', 'c',
+  default_options: [
+	'buildtype=debugoptimized',
+  ]
+)
+
+subdir('src')

src/meson.build

@@ -1,9 +1,3 @@
-project('GameNetworkingSockets', 'cpp', 'c',
-  default_options: [
-	'buildtype=debugoptimized',
-  ]
-)
-
 add_global_arguments('-fvisibility=hidden', language: 'cpp')
 add_global_arguments('-fvisibility-inlines-hidden', language: 'cpp')