Document secret-file in README and add example

README.md

@@ -84,6 +84,7 @@ act -v
   -q, --quiet                  disable logging of output from steps
   -r, --reuse                  reuse action containers to maintain state
   -s, --secret stringArray     secret to make available to actions with optional value (e.g. -s mysecret=foo or -s mysecret)
+      --secret-file            file with list of secrets to read from (e.g. --secret-file .secrets)
   -v, --verbose                verbose output
       --version                version for act
   -w, --watch                  watch the contents of the local repo and run when files change