update work on yaml readme
# Fixtures
-This directory contains fixtures for testing pip's resolver. The fixtures are written as yml files, with a convenient format that allows for specifying a custom index for temporary use.
+This directory contains fixtures for testing pip's resolver.
+The fixtures are written as `.yml` files, with a convenient format
+that allows for specifying a custom index for temporary use.
+
+The `.yml` files are organized in the following way. A `base` section
+which ...
<!-- TODO: Add a good description of the format and how it can be used. -->
