fix: change prepare to prepublishOnly + update readme

Author: aminya

README.md

@@ -6,7 +6,7 @@
 npm install minidump
 ```
 
-## Building
+## Building (for development)
 
 * `git clone --recurse-submodules https://github.com/electron/node-minidump`
 * `npm install`

package.json

@@ -20,7 +20,7 @@
     "test": "mocha test && standard",
     "build": "node build.js",
     "submodule": "git submodule update --recursive --init",
-    "prepare": "npm run submodule && npm run build",
+    "prepublishOnly": "npm run submodule && npm run build",
     "postinstall": "npm run build"
   },
   "devDependencies": {