message: Add link from basics to extending
diff: @@ -155,9 +155,7 @@ from the API.
  The idea of `urql` is that you can customise the caching behaviour amongst
  other things yourself, if needed.
  -<!-- TODO: add link to "Extending & Experimenting" -->
  -
  -[Read more about customising `urql` here](TODO.md)
  +[Read more about customising `urql` in the "Extending & Experimenting" section.](extending-and-experimenting.md)
  ## Subscriptions
  