Readme: Postgres version and environments

Found out that I couldn't pull the db since it's on a more recent
postgres.

README.md

@@ -12,6 +12,14 @@ website based on a custom CMS for listing links to dance events.
 preference for how to run Postgres locally we recommend
 [Postgres.app](https://postgresapp.com/).
 
+The postgres version you run locally should match the version used in production.
+If the app is deployed on Heroku (and you have access to it) you can check this with the
+[Heroku CLI](https://devcenter.heroku.com/articles/heroku-cli).
+
+For example, to get the version used on Swing Out London production:
+
+    heroku pg:info --app soldneu
+
 Other dependencies (including ruby) are listed in
 [`.tool-versions`](.tool-versions).
 
@@ -62,3 +70,13 @@ are not included in the .env.example file:
   - `CANONICAL_HOST` - all variations of the domain will be `301` redirected
     to this - eg. set it to `www.swingoutlondon.co.uk` to have
     `swingoutlondon.co.uk` redirect to www.
+
+## Environments
+
+There are currently 3 sites deployed using this codebase on heroku:
+
+- London Production: https://www.swingoutlondon.co.uk/ (heroku app: `soldneu`)
+- Bristol Production: https://www.swingoutbristol.org/ (heroku app:
+  `swing-out-bristol`)
+- Staging: https://soldn-staging.herokuapp.com/ (heroku app: `soldn-staging`)- used for testing changes.
+(Protected by HTTP Basic Authentication)