Documentation example: Haskell stack

There is an example for cabal. This is an example for stack. Both are
used in the haskell eco system.

README.md

@@ -64,6 +64,7 @@ See [Examples](examples.md) for a list of `actions/cache` implementations for us
 - [Elixir - Mix](./examples.md#elixir---mix)
 - [Go - Modules](./examples.md#go---modules)
 - [Haskell - Cabal](./examples.md#haskell---cabal)
+- [Haskell - Stack](./examples.md#haskell---stack)
 - [Java - Gradle](./examples.md#java---gradle)
 - [Java - Maven](./examples.md#java---maven)
 - [Node - npm](./examples.md#node---npm)

examples.md

@@ -5,6 +5,7 @@
   - [Elixir - Mix](#elixir---mix)
   - [Go - Modules](#go---modules)
   - [Haskell - Cabal](#haskell---cabal)
+  - [Haskell - Stack](#haskell---stack)
   - [Java - Gradle](#java---gradle)
   - [Java - Maven](#java---maven)
   - [Node - npm](#node---npm)
@@ -99,6 +100,19 @@ We cache the elements of the Cabal store separately, as the entirety of `~/.caba
     key: ${{ runner.os }}-${{ matrix.ghc }}-dist-newstyle
 ```
 
+## Haskell - Stack
+
+```yaml
+- uses: actions/cache@v1
+  name: Cache ~/.stack
+  with:
+    path: ~/.stack
+    key: ${{ runner.os }}-stack-${{ hashFiles('**/stack.yaml.lock') }}-${{ hashFiles('**/package.yaml') }}
+    restore-keys: |
+      ${{ runner.os }}-stack-${{ hashFiles('**/stack.yaml.lock') }}-
+      ${{ runner.os }}-stack-
+```
+
 ## Java - Gradle
 
 ```yaml