fix: Remove dead link to README.md (#148)

Release fails with the below otherwise:

```
error: failed to read `readme` file for package `yamux v0.11.0 (/home/xxx/rust-yamux/yamux)`
```

Author: mxinden

yamux/Cargo.toml

@@ -7,7 +7,6 @@ description = "Multiplexer over reliable, ordered connections"
 keywords = ["network", "protocol"]
 categories = ["network-programming"]
 repository = "https://github.com/paritytech/yamux"
-readme = "README.md"
 edition = "2018"
 
 [dependencies]