Avoid relative link in Readme for docker hub.

Don't use relative urls, they break on docker hub.

Author: Remco Bloemen

README.md

@@ -55,7 +55,7 @@ https://0x-org.gitbook.io/mesh.
 
 We love receiving contributions from the community :smile: If you are interested
 in helping develop 0x Mesh, please read the
-[Development Guide](CONTRIBUTING.md).
+[Development Guide](https://github.com/0xProject/0x-mesh/blob/master/CONTRIBUTING.md).
 If you are looking for a place to start, take a look at the
 [issues page](https://github.com/0xProject/0x-mesh/issues) and don't hesitate to
 [reach out to us on Discord](https://discord.gg/HF7fHwk).