README: Link to the distribution proposal

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

README.md

@@ -21,6 +21,7 @@ https://groups.google.com/a/opencontainers.org/forum/#!forum/tob (tob@opencontai
 ## Project Proposals
 
 * [Digest](https://github.com/opencontainers/tob/blob/master/proposals/digest.md)
+* [Distribution Spec](https://github.com/opencontainers/tob/blob/master/proposals/distribution.md)
 * [Image Format Spec](https://github.com/opencontainers/tob/tree/master/proposals/image-format)
 * [SELinux](https://github.com/opencontainers/tob/blob/master/proposals/selinux.md)
 * [Tools](https://github.com/opencontainers/tob/blob/master/proposals/tools.md)