add only_commits spec to .appveyor.yml

This commit adds an `only_commit` mapping to
`.appveyor.yml` that illustrates how to
límit CI builds to changes in code, tools
or `.appveyor.yml`.

The section has to be adapted if the
directory names are changed. Otherwise
some changes might not lead to CI-builds.

.appveyor.yml

@@ -172,6 +172,15 @@ install:
   #- sh: "[ -f ${HOME}/dlinstaller_env.sh ] && . ${HOME}/dlinstaller_env.sh || true"
 
 
+# only run the CI if there are code changes, tooling changes,
+# or changes in `.appveyor.yml`
+only_commits:
+  files:
+    - .appveyor.yml
+    - datalad_helloworld/
+    - tools/
+
+
 #before_build:
 #