💁 Adjust ospec bin def, add to README

Getting set up with ospec is deceptively simple, and adding just a bit
more documentation should help new users from falling into the trap of
overthinking the test config.  (I know that I ended up down a very
strange rabbit hole when pulling ospec into a project).

This also adjusts the bin definition in the package file so that we can
hopefully just use the `ospec` command instead of a folder ref.

ospec/package.json

@@ -9,6 +9,8 @@
   "keywords": [ "testing" ],
   "author": "Leo Horie <lhorie@hotmail.com>",
   "license": "MIT",
-  "bin": "./bin/ospec",
+  "bin": {
+    "ospec": "./bin/ospec"
+  },
   "repository": "lhorie/mithril.js#rewrite"
 }