Update README to include SKIP PR feature

[afalko]

We forgot to update our README when we added this flag: https://github.com/salesforce/dockerfile-image-update/blob/master/dockerfile-image-update/src/main/java/com/salesforce/dockerfileimageupdate/subcommands/impl/Parent.java#L49

As extra credit, we should create a unit test that reminds us to update README.
CC: @avimanyum

[justinharringa]

If we use labels properly (which we should/are) we could simply make sure that feature or enhancement PRs get a reminder poke in the PR. A vast majority of our PRs don't really introduce README-facing features.