Add instructions for setting a value only if the key does not exist

Use an empty prevValue to atomically test for the existence of a key before setting the value.
Merge pull request #272 from dmarti/patch-1

Add instructions for setting a value only if the key does not exist
feat(README): add an erlang library marshall-lee/etcd.erl
feat(README): add mailing list and IRC
Merge pull request #280 from philips/add-readme-contact

feat(README): add mailing list and IRC
fix(README): make contacts a list