make cargo install command more clear

see #2079 (comment)

Author: extrawurst

README.md

@@ -226,7 +226,8 @@ All contain a single binary file
 
 ### Cargo Install
 
-The simplest way to start playing around with `gitui` is to have `cargo` build and install it with `cargo install gitui`. If you are not familiar with rust and cargo: [Getting Started with Rust](https://doc.rust-lang.org/book/ch01-00-getting-started.html)
+The simplest way to start playing around with `gitui` is to have `cargo` build and install it with `cargo install gitui --locked`. If you are not familiar with rust and cargo: [Getting Started with Rust](https://doc.rust-lang.org/book/ch01-00-getting-started.html)
+
 ### Cargo Features
 #### trace-libgit
 enable `libgit2` tracing