node: add README (#7201)

plugins/node/README.md

+# node plugin
+
+To use it, add `node` to the plugins array of your zshrc file:
+```zsh
+plugins=(... node)
+```
+
+This plugin adds `node-docs` function that open specific section in [Node.js](https://nodejs.org) documentation (depending on the installed version).
+For example:
+
+```zsh
+# Opens https://nodejs.org/docs/latest-v10.x/api/fs.html
+$ node-docs fs
+# Opens https://nodejs.org/docs/latest-v10.x/api/path.html
+$ node-docs path 
+```