doc: update README

README.md

@@ -25,6 +25,20 @@
 - [libcli](https://git.lohic.dev/loic/libcli)
 - [libft_ssl](https://git.lohic.dev/loic/libft_ssl)
 
+## Usage
+
+```
+ft_ssl {md5|sha256|whirlpool} [-pqr] [-s string] [file...]
+
+Options:
+  -h, --help                     Display this help and exit
+  -V, --version                  Display version information and exit
+  -p, --stdin                    Echo stdin to stdout and append checksum to output
+  -q, --quiet                    Quiet mode: only print the digest
+  -r, --reverse                  Reverse the output format (digest then filename)
+  -s, --string          <STR>    Hash a string
+```
+
 ## License
 
 This project is licensed under the GNU General Public License v3.0 - see the [LICENSE](LICENSE) file for details.