Try to add Gitpod (kowainik#452)

* Create .gitpod.yml

* Readme: add Gitpod

* Update README.md

* Update .gitpod.yml

* Update README.md

* Update README.md

* Update .gitpod.yml

* Update .gitpod.yml

* Update README.md

* add button

Co-authored-by: Geoffrey Huntley <[email protected]>

* Update README.md

Co-authored-by: Geoffrey Huntley <[email protected]>

Author: imba-tjd

.gitpod.yml

@@ -0,0 +1,35 @@
+# from https://github.com/haskell/haskell-language-server/blob/master/.gitpod.yml
+tasks:
+  - before: |
+      # Only the /workspace folder is persistent
+      export XDG_DATA_HOME=/workspace/.local/share
+      export XDG_CONFIG_HOME=/workspace/.local/config
+      export XDG_STATE_HOME=/workspace/.local/state
+      export XDG_CACHE_HOME=/workspace/.cache
+      export CABAL_DIR=/workspace/.cabal
+      export STACK_ROOT=/workspace/.stack
+
+      # install ghcup, ghc and cabal
+      export GHCUP_INSTALL_BASE_PREFIX=/workspace
+      export BOOTSTRAP_HASKELL_NONINTERACTIVE=1
+      export BOOTSTRAP_HASKELL_MINIMAL=1
+      curl --proto '=https' --tlsv1.2 -sSf https://get-ghcup.haskell.org | sh
+      /workspace/.ghcup/bin/ghcup install ghc --set
+      /workspace/.ghcup/bin/ghcup install cabal
+
+      # Add ghcup binaries to the PATH since VSCode does not see 'source .ghcup/env'
+      pushd /usr/local/bin
+      sudo ln -s /workspace/.ghcup/bin/* /usr/local/bin
+      popd
+
+      # Fix the Cabal dir since VSCode does not see CABAL_DIR 
+      cabal update
+      echo "Symlinking /workspace/.cabal to ~/.cabal"
+      ln -s /workspace/.cabal ~
+    init: | 
+      cabal update
+
+vscode:
+  extensions:
+    - haskell.haskell
+    - justusadam.language-haskell

README.md

@@ -200,6 +200,18 @@ If you don't have any IDE preferences, we recommend installing
 [Haskell plugin](https://marketplace.visualstudio.com/items?itemName=haskell.haskell).
 The mentioned plugin would give you everything required to immediately start coding with Haskell.
 
+### Gitpod
+
+[![Open in Gitpod](https://gitpod.io/button/open-in-gitpod.svg)](https://gitpod.io/#https://github.com/kowainik/learn4haskell)
+
+[Gitpod](https://www.gitpod.io/) is a VSCode-based Web IDE.
+With it, you can get a Haskell environment out-of-the-box.
+It's free to use up to 50 hours per month.
+
+Just prepend `gitpod.io#` to your repo URL and you are ready to go.
+It will take some time to initialize the workspace for the first time it opens.
+It only keeps changes under `/workspace`, and it will be deleted after a period of inactivity unless it's pinned.
+
 ### How to develop
 
 The course assumes that you install Haskell tooling (GHC and Cabal), edit code