Create CONTRIBUTING.md

Author: kevinongko

CONTRIBUTING.md

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+# Contributing guidelines
+When contributing to this repository, please first discuss the change you wish to make via issue, email, or any other method with the owners of this repository before making a change.
+
+## Proposing a change
+If you intend to change the public API, or make any non-trivial changes to the implementation, we recommend filing an issue. This lets us reach an agreement on your proposal before you put significant effort into it.
+
+## Javascript style guides
+JavaScript must adhere to [Vue.Js official recommended style](https://github.com/vuejs/eslint-plugin-vue).
+
+## DocBlock
+Every function, methods, computed properties, props should have a docblock to explain what it does
+
+example:
+```js
+    /**
+     * Check provided value againts maximum allowed.
+     * @param {Number} value
+     * @return {Boolean}
+     */
+    checkMaxValue (value) {
+      if (this.max) {
+        if (value <= this.maxValue) return false
+        return true
+      }
+      return false
+    },
+```
+
+## Test
+- Make sure current tests is all green :white_check_mark:
+- If you've added code that should be tested, add tests!
+- Any new feature or changes should include tests for it.
+
+## Readme
+Update the [README.md](https://github.com/kevinongko/vue-numeric/blob/master/README.md) with details of changes.