github: suppress running CI only for documentation

For updating documentation or release task, running GitHub Actions
is waste of resource.

https://docs.github.com/en/actions/writing-workflows/workflow-syntax-for-github-actions#onpushpull_requestpull_request_targetpathspaths-ignore

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

Author: kenhys

.github/workflows/test.yml

@@ -3,8 +3,13 @@ name: Test
 on:
   push:
     branches: [master]
+    paths-ignore:
+      - '*.md'
+      - 'lib/fluent/version.rb'
   pull_request:
     branches: [master]
+      - '*.md'
+      - 'lib/fluent/version.rb'
 
 permissions: read-all