Add recipe subfolder info to developer documentation

If #477 is merged, developers will be expected to provide both the recipe name and filepath if either of these are not of the expected format.

Expected format if string added to `flow.cylc` is "`thing`":
recipe name: `recipe_thing.yml`
recipe filepath in ESMValTool: `ESMValTool/esmvaltool/recipes/recipe_thing.yml`

This should be documented for developers adding new recipes.

Locate the developer documentation covering how to add new recipes; the issue does not name the documentation file. Document that a recipe added through flow.cylc must use the expected recipe name and ESMValTool filepath format, using the recipe_thing.yml example. Done means the convention is stated clearly for developers and the reference to #477 is accurate.

Written by the indexing model from the issue text.