add usage

README.md

@@ -2,10 +2,15 @@
 
 Digest tool written in Rust programming language
 
+Usage:
+```shell
+$ digest -a sha256 LICENSE 
+SHA256: e36f811f31f3ad9d2ba68230258747bcb868ab59e8640fab3733fbe179bb7cf4
+```
 
 Install:
 ```shell
-cargo install --git https://git.hatter.ink/hatter/digest.git
+$ cargo install --git https://git.hatter.ink/hatter/digest.git
 ```
 
 Supported algorithms:<br>