Add description to package specification

If there is no explicit description, the registry appears to use the
first two lines of the documentation as an implicit description.

package.json

@@ -1,6 +1,7 @@
 {
   "name": "parity-pmd",
   "version": "0.1.0",
+  "description": "Parity's native market data protocol",
   "author": "Jussi Virtanen",
   "license": "Apache-2.0",
   "repository": "jvirtanen/node-parity-pmd",