Fully automate dev setup with Gitpod

This commit implements a fully-automated development setup using Gitpod.io, an
online IDE for GitLab, GitHub, and Bitbucket that enables Dev-Environments-As-Code.
This makes it easy for anyone to get a ready-to-code workspace for any branch,
issue or pull request almost instantly with a single click.

.gitpod.yml

@@ -0,0 +1,3 @@
+tasks:
+  - init: cargo build
+    command: cargo watch -x run

README.md

@@ -1,3 +1,5 @@
+[![Gitpod ready-to-code](https://img.shields.io/badge/Gitpod-ready--to--code-blue?logo=gitpod)](https://gitpod.io/#https://github.com/nwtgck/piping-server-rust)
+
 # piping-server
 [![CI](https://github.com/nwtgck/piping-server-rust/workflows/CI/badge.svg)](https://github.com/nwtgck/piping-server-rust/actions) [![CircleCI](https://circleci.com/gh/nwtgck/piping-server-rust.svg?style=shield)](https://circleci.com/gh/nwtgck/piping-server-rust) [![Docker Image Size (latest by date)](https://img.shields.io/docker/image-size/nwtgck/piping-server-rust)](https://hub.docker.com/r/nwtgck/piping-server-rust)