docs: add package installation instructions

Author: lingbopro

README.md

@@ -9,20 +9,12 @@
 
 WIP
 
-<!-- Using npm / yarn / pnpm:
+Using npm:
 
 ```console
 npm install --save-dev rollup-plugin-custom-import
 ```
 
-```console
-yarn add --dev rollup-plugin-custom-import
-```
-
-```console
-pnpm add --save-dev rollup-plugin-custom-import
-``` -->
-
 ## Usage
 
 Create a `rollup.config.js` [configuration file](https://www.rollupjs.org/command-line-interface/#configuration-files) and import the plugin: