Add commit pinning example to readme
Add commit pinning example to readme

Signed-off-by: Charles Hansen <c376c0f4afe7933d7fe8f7dfb7f868092051f165@pivotal.io>
Co-authored-by: Chris Tarazi <52172d1cc353fd06218578cc1e854d12546b9ed2@pivotal.io>

concourse/git-resource