1.0: Finalised for release

Updating the README.md to bring it ready for wider release:
* Removed some points that were more implementation details
* Added Contributions section
* Simplified the usage section
* Expanded initial explanation
* Fixed steam launch command to allow the overlay to still work

Others:
* Mark as 1.0
* Update the license file for 2018

meson.build

@@ -2,7 +2,7 @@ project(
     'gamemode',
     'c',
     default_options : ['c_std=c11'],
-    version: '0.3',
+    version: '1.0',
     license: 'BSD',
 )