Show how to set BUNDLE_GEMFILE in the README

Author: eregon

README.md

@@ -136,6 +136,23 @@ This caching speeds up installing gems significantly and avoids too many request
 It needs a `Gemfile` (or `$BUNDLE_GEMFILE` or `gems.rb`) under the [`working-directory`](#working-directory).  
 If there is a `Gemfile.lock` (or `$BUNDLE_GEMFILE.lock` or `gems.locked`), `bundle config --local deployment true` is used.
 
+To use a `Gemfile` which is not at the root, set `BUNDLE_GEMFILE` in the `env` at the job level, so it is set for all steps:
+```yaml
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    env:
+      BUNDLE_GEMFILE: subdir/Gemfile
+    steps:
+    - uses: actions/checkout@v2
+    - uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: 2.6
+        bundler-cache: true
+    - run: bundle exec rake
+```
+
 To perform caching, this action will use `bundle config --local path vendor/bundle`.  
 Therefore, the Bundler `path` should not be changed in your workflow for the cache to work (no `bundle config path`).