Readme: Mention shell completions (#10)

Author: tangrufus

README.md

@@ -219,6 +219,11 @@ Finally, install the package:
 sudo dpkg -i composer-semver_Linux_arm64.deb
 ```
 
+For shell completions, follow the instructions from:
+```sh
+composer-semver completion --help
+```
+
 ## Manual Binary
 
 > [!WARNING]
@@ -244,6 +249,11 @@ tar -xvf 'composer-semver_Darwin_arm64.tar.gz'
 mv composer-semver /usr/local/bin
 ```
 
+For shell completions, follow the instructions from:
+```sh
+composer-semver completion --help
+```
+
 ## Alternatives
 
 - [ComVer](https://github.com/typisttech/comver)