Mention Argos in leading paragraph

One of the first things that people will probably want to know about this
project is how the actual translation work. Therefore, add mention of
Argo to the leading paragraph of the readme.

README.md

@@ -4,7 +4,7 @@
 
 [![Python versions](https://img.shields.io/pypi/pyversions/libretranslate)](https://pypi.org/project/libretranslate) [![Run tests](https://github.com/uav4geo/LibreTranslate/workflows/Run%20tests/badge.svg)](https://github.com/uav4geo/LibreTranslate/actions?query=workflow%3A%22Run+tests%22) [![Publish to DockerHub](https://github.com/uav4geo/LibreTranslate/workflows/Publish%20to%20DockerHub/badge.svg)](https://hub.docker.com/r/libretranslate/libretranslate) [![Publish to GitHub Container Registry](https://github.com/uav4geo/LibreTranslate/workflows/Publish%20to%20GitHub%20Container%20Registry/badge.svg)](https://github.com/uav4geo/LibreTranslate/actions?query=workflow%3A%22Publish+to+GitHub+Container+Registry%22) [![Awesome Humane Tech](https://raw.githubusercontent.com/humanetech-community/awesome-humane-tech/main/humane-tech-badge.svg?sanitize=true)](https://github.com/humanetech-community/awesome-humane-tech)
 
-Free and Open Source Machine Translation API, entirely self-hosted. Unlike other APIs, it doesn't rely on proprietary providers such as Google or Azure to perform translations.
+Free and Open Source Machine Translation API, entirely self-hosted. Unlike other APIs, it doesn't rely on proprietary providers such as Google or Azure to perform translations. Instead, its translation engine is powered by the open source [Argos Translate][argo] library.
 
 ![image](https://user-images.githubusercontent.com/1951843/121782367-23f90080-cb77-11eb-87fd-ed23a21b730f.png)
 
@@ -276,8 +276,10 @@ To get a HTTPS subdomain certificate, install `certbot` (snap), run `sudo certbo
 
 ## Credits
 
-This work is largely possible thanks to [Argos Translate](https://github.com/argosopentech/argos-translate), which powers the translation engine.
+This work is largely possible thanks to [Argos Translate][argo], which powers the translation engine.
 
 ## License
 
 [GNU Affero General Public License v3](https://www.gnu.org/licenses/agpl-3.0.en.html)
+
+[argo]: https://github.com/argosopentech/argos-translate