Initial project setup

Partly ported from actions/setup-ruby

README.md

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+# setup-haskell
+
+<p align="left">
+  <a href="https://github.com/actions/setup-haskell"><img alt="GitHub Actions status" src="https://github.com/actions/setup-haskell/workflows/Main%20workflow/badge.svg"></a>
+</p>
+
+This action sets up a Haskell environment for use in actions by:
+
+- optionally installing a version of ghc and cabal and adding to PATH. Note that this action only uses versions of ghc and cabal already installed in the cache. The action will fail if no matching versions are found.
+- registering problem matchers for error output
+
+# Usage
+
+See [action.yml](action.yml)
+
+Basic:
+
+``` yaml
+steps:
+- uses: actions/checkout@master
+- uses: actions/setup-haskell@v1
+  with:
+    ghc-version: '8.6.5' # Version range or exact version of ghc to use, using semvers version range syntax.
+- run: runghc Hello.hs
+```
+
+Matrix Testing:
+
+``` yaml
+jobs:
+  build:
+    runs-on: ubuntu-16.04
+    strategy:
+      matrix:
+        ghc: [ '8.2.2', '8.6.5' ]
+    name: Haskell GHC ${{ matrix.ghc }} sample
+    steps:
+      - uses: actions/checkout@master
+      - name: Setup Haskell
+        uses: actions/setup-haskell@v1
+        with:
+          ghc-version: ${{ matrix.ghc }}
+      - run: runghc Hello.hs
+```
+
+# License
+
+The scripts and documentation in this project are released under the [MIT License](LICENSE)
+
+# Contributions
+
+Contributions are welcome!  See [Contributor's Guide](docs/contributors.md)