update readme

Author: bothness

README.md

@@ -1,3 +1,9 @@
-**Legacy Github Pages previews and redirects**
+# onsvisual.github.io
 
-This repo is being retained on the ONSvisual Github org in order to maintain redirects of Github Pages sites to their new locations on the ONSdigital org.
+This repo has been retained on the ONSvisual Github organisation in order to maintain redirects of Github Pages sites to their new locations on the ONSdigital organisation.
+
+## How redirects work
+
+For repos moved from one organisation to another, the actual repo page on Github should redirect automatically. Eg. [github.com/ONSvisual/uk-topojson](https://github.com/ONSvisual/uk-topojson) redirects to [github.com/ONSdigital/uk-topojson](https://github.com/ONSdigital/uk-topojson)
+
+However, associated Github Pages sites do not automatically redirect. Therefore, for repos that have associated Pages sites, we create a folder in this repo containing an index.html file that redirects to the new site. To follow on from the above example, this allows us to redirect [onsvisual.github.io/uk-topojson](https://onsvisual.github.io/uk-topojson/) to [onsdigital.github.io/uk-topojson](https://onsdigital.github.io/uk-topojson/)