mention weblate in README

Link to the Grist project on weblate, as a starting point for anyone
who'd like to contribute translations.

README.md

@@ -146,6 +146,16 @@ There are a lot of ways to do this, including [SAML and forward authentication](
 Grist has been tested with [Authentik](https://goauthentik.io/), [Auth0](https://auth0.com/),
 and Google/Microsoft sign-ins via [Dex](https://dexidp.io/).
 
+## Translations
+
+We use [Weblate](https://hosted.weblate.org/engage/grist/) to manage translations.
+Thanks to everyone who is pitching in. Thanks especially to the ANCT developers who
+did the hard work of making a good chunk of the application localizable. Merci bien!
+
+<a href="https://hosted.weblate.org/engage/grist/">
+<img src="https://hosted.weblate.org/widgets/grist/-/open-graph.png" alt="Translation status" />
+</a>
+
 ## Why free and open source software
 
 This repository, [grist-core](https://github.com/gristlabs/grist-core), is maintained by Grist