docs: npm home nyc goes to github master branch README (#1201)

This causes confusion among users who think they're reading about the
latest releaase.  It is better that we send users to istanbul.js.org for
documentation.

Issue #1200

package.json

@@ -99,6 +99,7 @@
   "engines": {
     "node": ">=8"
   },
+  "homepage": "https://istanbul.js.org/",
   "repository": {
     "type": "git",
     "url": "git@github.com:istanbuljs/nyc.git"