[add] npm section (#32)

Author: get-me-power

README.md

@@ -17,10 +17,18 @@
 
 ## Install
 
+**For yarn**
+
 ``` sh
 yarn global add vim-language-server
 ```
 
+**For npm**
+
+```sh
+npm install -g vim-language-server
+```
+
 **For coc.nvim user** install coc extension:
 
 ``` vim