Infra: Add note on finding the RTD preview (#2551)

It's very useful but a bit buried. Hopefully this will help more people find it.

Co-authored-by: CAM Gerlach <CAM.Gerlach@Gerlach.CAM>

README.rst

@@ -24,6 +24,18 @@ generation to fail, or result in major rendering defects relative to what you
 intend.
 
 
+Browse the ReadTheDocs preview
+------------------------------
+
+For every PR, we automatically create a preview of the rendered PEPs using
+`ReadTheDocs <https://readthedocs.org/>`_.
+You can find it in the merge box at the bottom of the PR page:
+
+1. Click "Show all checks" to expand the checks section
+2. Find the line for ``docs/readthedocs.org:pep-previews``
+3. Click on "Details" to the right
+
+
 Render PEPs locally
 -------------------