docs(core): complete README

Author: posva

packages/@posva/vuefire-core/README.md

@@ -1,11 +1,74 @@
-# `@posva/vuefire-core`
+# Vuefire Core
 
-> TODO: description
+> Core logic used for vuefire and vuexfire
 
-## Usage
+This library is intended for internal usage. You are free to use it to create your own plugins but keep in mind that the main target is a Vue plugin
+
+## Installation
 
+```sh
+npm i @posva/vuefire-core
 ```
-const vuefireCore = require('@posva/vuefire-core');
 
-// TODO: DEMONSTRATE API
+## Usage
+
+```js
+import { bindCollection, bindDocument, walkSet } from '@posva/vuefire-core'
+
+// create an object of operations
+const ops = {
+  set: (target, path, value) => walkSet(target, path, value),
+  add: (array, index, data) => array.splice(index, 0, data),
+  remove: (array, index) => array.splice(index, 1),
+}
+const vm = new Vue({
+  // options
+})
+
+const resolve = data => {
+  console.log('reference bound:', data)
+}
+
+const reject = err => {
+  console.log('error binding reference:', err)
+}
+
+// unbind is a function that tears down all listeners
+const unbindItems = bindCollection(
+  {
+    // vm could be just an object
+    vm,
+    // key set on vm
+    key: 'items',
+    ops,
+    collection: db.collection('items'),
+    // this is to enable Promise based APIs
+    // callback on success
+    resolve,
+    // callback on error
+    reject,
+  },
+  // default options
+  {
+    maxRefDepth: 2,
+  }
+)
+
+const unbindItem = bindDocument(
+  {
+    // same options as bindCollection except for collection -> document
+    document: db.collection('items').doc('0'),
+  },
+  options
+)
+
+unbindItems()
+bindCollection({
+  // bind a different collection
+  key: 'items',
+})
 ```
+
+## License
+
+[MIT](http://opensource.org/licenses/MIT)