docs: update cache step example for yarn

https://github.com/actions/cache/blob/master/examples.md#node---yarn
https://github.com/actions/cache/pull/70

README.md

@@ -486,11 +486,15 @@ jobs:
       with:
         node-version: '10.x'
 
+    - name: Get yarn cache
+      id: yarn-cache
+      run: echo "::set-output name=dir::$(yarn cache dir)"
+
     - name: Cache dependencies
       uses: actions/cache@v1
       with:
-        path: ~/.cache/yarn
-        key: ${{ runner.os }}-yarn-${{ hashFiles(format('{0}{1}', github.workspace, '/yarn.lock')) }}
+        path: ${{ steps.yarn-cache.outputs.dir }}
+        key: ${{ runner.os }}-yarn-${{ hashFiles('**/yarn.lock') }}
         restore-keys: |
           ${{ runner.os }}-yarn-