Fix README

Could not use this action copying from README, had to add the action
version.

README.md

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-      - uses: everlytic/branch-merge
+      - uses: everlytic/branch-merge@1.0.2
         with:
           github_token: ${{ github.token }}
           source_ref: ${{ github.ref }}