Add metadata to .app file in order to publish Syn to hex.pm.

Also, add rebar.lock for rebar3 compatibility.

rebar.lock

@@ -0,0 +1 @@
+[].

src/syn.app.src

@@ -13,6 +13,10 @@
             mnesia
         ]},
         {mod, {syn_app, []}},
-        {env, []}
+        {env, []},
+
+        {maintainers, ["Roberto Ostinelli"]},
+        {licenses, ["MIT"]},
+        {links, [{"Github", "https://github.com/ostinelli/syn"}]}
     ]
 }.