Add results-from feature flag to config-feature-flags.yaml

Prior to this the feature flag `results-from` was only mentioned in
the documentation and missing from the config-feature-flags.yaml file.
This PR fixes it and sets it to "termination-message" which is the
default value.

config/config-feature-flags.yaml

@@ -91,3 +91,8 @@ data:
   # If set to "none", then Tekton will not have non-falsifiable provenance.
   # This is an experimental feature and thus should still be considered an alpha feature.
   enforce-nonfalsifiablity: "none"
+  # Setting this flag will determine how Tekton pipelines will handle extracting results from the task.
+  # Acceptable values are "termination-message" or "sidecar-logs".
+  # "sidecar-logs" is an experimental feature and thus should still be considered
+  # an alpha feature.
+  results-from: "termination-message"