update manifest with path to README.md

Fixes: #80

Signed-off-by: Andreea Florescu <fandree@amazon.com>

Author: andreeaflorescu

Cargo.toml

@@ -5,6 +5,7 @@ description = "Safe abstractions for accessing the VM physical memory"
 keywords = ["memory"]
 authors = ["Liu Jiang <gerry@linux.alibaba.com>"]
 repository = "https://github.com/rust-vmm/vm-memory"
+readme = "README.md"
 license = "Apache-2.0 OR BSD-3-Clause"
 edition = "2018"