locked flag added to building section. (paritytech#1723)

As described in paritytech#1717 running `cargo install` without `--locked` flag may end up with build fails.

Author: Nikita Puzankov

README.md

@@ -67,7 +67,7 @@ dnf install polkadot
 If you want to install Polkadot in your PATH, you can do so with with:
 
 ```bash
-cargo install --git https://github.com/paritytech/polkadot --tag <version> polkadot
+cargo install --git https://github.com/paritytech/polkadot --tag <version> polkadot --locked
 ```
 
 ### Build from Source