add license to package.json

Author: Paveł Tyślacki

package.json

@@ -3,6 +3,7 @@
   "version": "0.0.3",
   "description": "Combine glyph (SDF) PBFs on the fly.",
   "main": "index.js",
+  "license": "BSD-2-Clause",
   "dependencies": {
     "protocol-buffers": "^2.4.7"
   },