Fully automate dev setup with Gitpod

This commit implements a fully-automated development setup using Gitpod.io, an
online IDE for GitHub and GitLab 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.Dockerfile

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+FROM gitpod/workspace-full
+
+# Install custom tools, runtimes, etc.
+# For example "bastet", a command-line tetris clone:
+# RUN brew install bastet
+#
+# More information: https://www.gitpod.io/docs/config-docker/

.gitpod.yml

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+image:
+  file: .gitpod.Dockerfile
+
+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/rust-lang/rustlings)
+
 ![crab pet](https://i.imgur.com/LbZJgmm.gif)
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