Add docs/pipeline-api.md.backup to .gitignore

It'd be nice to get rid of this file entirely, but it's a result of 8381d0d and I'm not sure how to tweak `sed` args to not generate a backup file but also work right on Macs...

Signed-off-by: Andrew Bayer <andrew.bayer@gmail.com>

.gitignore

@@ -57,3 +57,6 @@ test/pullrequest/pullrequest-init
 # Temporary GOPATH used during code gen if user's Tekton checkout is
 # not already in GOPATH.
 .gopath
+
+# Backup of API reference docs generated by sed
+docs/pipeline-api.md.backup

hack/update-reference-docs.sh

@@ -23,4 +23,4 @@ go run github.com/ahmetb/gen-crd-api-reference-docs \
     -api-dir "github.com/tektoncd/pipeline/pkg/apis" \
     -template-dir "./hack/reference-docs-template" \
     -out-file "./docs/pipeline-api.md"
-sed -i".backup" '1s/^/<!--\n---\ntitle: Pipeline API\nlinkTitle: Pipeline API\nweight: 1000\n---\n-->\n\n/' ./docs/pipeline-api.md
+sed -i".backup" '1s/^/<!--\n---\ntitle: Pipeline API\nlinkTitle: Pipeline API\nweight: 1000\n---\n-->\n\n/' ./docs/pipeline-api.md && rm docs/pipeline-api.md.backup