Upgrade conf dependency

.travis.yml

@@ -1,5 +1,6 @@
+os:
+  - osx
 language: node_js
 node_js:
   - '10'
   - '8'
-  - '6'

package.json

@@ -32,11 +32,11 @@
 		"save"
 	],
 	"dependencies": {
-		"conf": "^3.0.0"
+		"conf": "^4.0.0"
 	},
 	"devDependencies": {
-		"ava": "^1.3.1",
-		"electron": "^4.1.1",
+		"ava": "^1.4.1",
+		"electron": "^4.1.3",
 		"execa": "^1.0.0",
 		"xo": "^0.24.0"
 	},

readme.md

@@ -57,7 +57,48 @@ Type: `Object`
 
 Type: `Object`
 
-Default data.
+Default values for the store items.
+
+**Note:** The values in `defaults` will overwrite the `default` key in the `schema` option.
+
+#### schema
+
+type: `Object`
+
+[JSON Schema](https://json-schema.org) to validate your config data.
+
+Under the hood, the JSON Schema validator [ajv](https://github.com/epoberezkin/ajv) is used to validate your config. We use [JSON Schema draft-07](http://json-schema.org/latest/json-schema-validation.html) and support all [validation keywords](https://github.com/epoberezkin/ajv/blob/master/KEYWORDS.md) and [formats](https://github.com/epoberezkin/ajv#formats).
+
+You should define your schema as an object where each key is the name of your data's property and each value is a JSON schema used to validate that property. See more [here](https://json-schema.org/understanding-json-schema/reference/object.html#properties).
+
+Example:
+
+```js
+const Store = require('electron-store');
+
+const schema = {
+	foo: {
+		type: 'number',
+		maximum: 100,
+		minimum: 1,
+		default: 50
+	},
+	bar: {
+		type: 'string',
+		format: 'url'
+	}
+};
+
+const store = new Store({schema});
+
+console.log(store.get('foo'));
+//=> 50
+
+store.set('foo', '1');
+// [Error: Config schema violation: `foo` should be number]
+```
+
+**Note:** The `default` value will be overwritten by the `defaults` option if set.
 
 #### name