Add gitpod config

this commit adds support for Gitpod.io, a free automated
dev environment that makes contributing and generally working on GitHub
projects much easier. It allows anyone to start a ready-to-code dev
environment for any branch, issue and pull request with a single click.

Author: anikethsaha

.gitpod.yml

@@ -0,0 +1,2 @@
+tasks:
+  - init: npm install

README.md

@@ -93,6 +93,10 @@ docsify serve <path> [--open false] [--port 3000]
 
 Please see the [Contributing Guidelines](./CONTRIBUTING.md)
 
+## Contribution
+
+[![Open in Gitpod](https://gitpod.io/button/open-in-gitpod.svg)](https://gitpod.io/#https://github.com/docsifyjs/docsify-cli)
+
 ## License
 
 [MIT](./LICENSE)