docs: explicit use of environment.yml

Co-authored-by: jaimergp <[email protected]>

README.md

@@ -115,7 +115,7 @@ If you don’t already have a conda-based environment manager, install one. For
 To develop `conda-smithy`, use said environment manager and create an environment based on the included `environment.yml` file.
 
 ```sh
-$ conda env create
+$ conda env create -f environment.yml
 ```
 
 Then activate the environment, check that it works and install the dependencies: