Use julia-actions/cache in README example (#99)

README.md

@@ -18,6 +18,11 @@ on:
       - main
   pull_request:
 
+# needed to allow julia-actions/cache to delete old caches that it has created
+permissions:
+  actions: write
+  contents: read
+
 jobs:
   test:
     runs-on: ${{ matrix.os }}
@@ -36,6 +41,7 @@ jobs:
         with:
           version: ${{ matrix.julia-version }}
           arch: ${{ matrix.julia-arch }}
+      - uses: julia-actions/cache@v1
       - uses: julia-actions/julia-buildpkg@v1
       - uses: julia-actions/julia-runtest@v1
         # with: