docs: update README.md - remove --save instruction (#735)

According to https://blog.npmjs.org/post/161081169345/v500.html . The --save options is default since npm version 5

Author: zamplyy

README.md

@@ -17,19 +17,19 @@ Not ready yet? Read documentation for older versions [here →](https://github.c
 ### npm & ESM
 
 ```bash
-$ npm install --save echarts vue-echarts
+$ npm install echarts vue-echarts
 ```
 
 To make `vue-echarts` work for _Vue 2_ (<2.7.0), you need to have `@vue/composition-api` installed:
 
 ```sh
-npm i --save @vue/composition-api
+npm i @vue/composition-api
 ```
 
 If you are using _NuxtJS_ on top of _Vue 2_, you'll also need `@nuxtjs/composition-api`:
 
 ```sh
-npm i --save @nuxtjs/composition-api
+npm i @nuxtjs/composition-api
 ```
 
 And then add `'@nuxtjs/composition-api/module'` in the `buildModules` option in your `nuxt.config.js`.

README.zh-Hans.md

@@ -17,19 +17,19 @@
 ### npm & ESM
 
 ```bash
-$ npm install --save echarts vue-echarts
+$ npm install echarts vue-echarts
 ```
 
 要在 _Vue 2_（<2.7.0）下使用 `vue-echarts`，需要确保 `@vue/composition-api` 已经安装：
 
 ```sh
-npm i --save @vue/composition-api
+npm i @vue/composition-api
 ```
 
 如果你在使用基于 _Vue 2_ 的 _NuxtJS_，那么还需要安装 `@nuxtjs/composition-api`：
 
 ```sh
-npm i --save @nuxtjs/composition-api
+npm i @nuxtjs/composition-api
 ```
 
 然后在 `nuxt.config.js` 的 `buildModules` 选项中添加 `'@nuxtjs/composition-api/module'`。