Clarify that git rm -r --cached only needs to run during migration

Co-Authored-By: Dongdong Tian <seisman.info@gmail.com>

Author: weiji14

CONTRIBUTING.md

@@ -483,8 +483,8 @@ summarized as follows:
     mv baseline/*.png pygmt/tests/baseline/
 
     # Generate hash for baseline image and stage the *.dvc file in git
+    git rm -r --cached 'pygmt/tests/baseline/test_logo.png'  # only run if migrating existing image from git to dvc
     dvc add pygmt/tests/baseline/test_logo.png
-    git rm -r --cached 'pygmt/tests/baseline/test_logo.png'  # optional
     git add pygmt/tests/baseline/test_logo.png.dvc
 
     # Commit changes and push to both the git and dvc remotes