API Platform: add a post install message

api-platform/core/2.1/post-install.txt

@@ -0,0 +1,12 @@
+<bg=blue;fg=white>              </>
+<bg=blue;fg=white> What's next? </>
+<bg=blue;fg=white>              </>
+
+  * Your API is almost ready:
+    1. Create your first API resource in <info>src/Entity</info>;
+    2. Go to <info>/api</info> to browse your API
+
+  * To enable the GraphQL support, run <comment>composer require webonyx/graphql-php</>,
+    then browse <info>/api/graphql</info>.
+
+  * <fg=blue>Read</> the documentation at <comment>https://api-platform.com/docs</>