docs(README): update install instructions

recent versions of npm save packages to `package.json` by default. 
The `--save` flag is not required anymore.

Author: gruhn

README.md

@@ -103,7 +103,7 @@ The newest API these components depend on is the [FileReader API](https://canius
 # Installation
 
 ```bash
-npm install --save vue-qrcode-reader
+npm install vue-qrcode-reader
 ```
 or with Yarn: