Add note about minimum docker-compose version (mastodon#1264)

Add a note to ensure users are using the right version of docker-compose.

README.md

@@ -67,7 +67,7 @@ Consult the example configuration file, `.env.production.sample` for the full li
 
 [![](https://images.microbadger.com/badges/version/gargron/mastodon.svg)](https://microbadger.com/images/gargron/mastodon "Get your own version badge on microbadger.com") [![](https://images.microbadger.com/badges/image/gargron/mastodon.svg)](https://microbadger.com/images/gargron/mastodon "Get your own image badge on microbadger.com")
 
-The project now includes a `Dockerfile` and a `docker-compose.yml`. You need to turn `.env.production.sample` into `.env.production` with all the variables set before you can:
+The project now includes a `Dockerfile` and a `docker-compose.yml` file (which requires at least docker-compose version `1.10.0`). You need to turn `.env.production.sample` into `.env.production` with all the variables set before you can:
 
     docker-compose build