Update README.md to use relative links
Update README.md to use relative links

GitHub supports relative links now:
https://help.github.com/articles/relative-links-in-readmes
dain/swift,rojanu/swift,electrum/swift,coderczp/swift,coderczp/swift,bgould/swift,glybuaa/swift,andrewcox/swift,airlift/drift,karryoberes/swift,rojanu/swift,dain/swift,glybuaa/swift,alandau/swift,adriancole/swift,karryoberes/swift,electrum/drift,facebook/swift,dain/drift,bgould/swift,adriancole/swift,facebook/swift,andrewcox/swift,electrum/swift