Added instructions for using cocoapods beta
Added instructions for using cocoapods beta

Added instructions from https://github.com/getsentry/raven-swift/issues/3
hankbao/raven-swift,hankbao/raven-swift,eriksargent/raven-swift,eriksargent/raven-swift,hankbao/raven-swift,eriksargent/raven-swift