add guide of signing CLA (#881)

* add guide of signing CLA

* add oxford comma

.github/pull_request_template.md

@@ -1,8 +1,12 @@
 <!--Thanks for your contribution to TiDB Operator documentation. See [CONTRIBUTING](https://github.com/pingcap/community/blob/master/CONTRIBUTING.md) before filing this pull request (PR).-->
 
-### What is changed, added or deleted? (Required)
+### First-time contributors' checklist <!--Remove this section if you're not a first-time contributor.-->
 
-<!--Tell us what you did and why. This is important to help reviewers and community members understand your PR.-->
+- [ ] I've signed [**Contributor License Agreement**](https://cla-assistant.io/pingcap/docs-tidb-operator) that's required for repo owners to accept my contribution.
+
+### What is changed, added, or deleted? (Required)
+
+<!--Tell us what you did and why.-->
 
 ### Which TiDB Operator version(s) do your changes apply to? (Required)
 
@@ -12,8 +16,6 @@
 - [ ] v1.1 (TiDB Operator 1.1 versions)
 - [ ] v1.0 (TiDB Operator 1.0 versions)
 
-<!--**If you select two or more versions from above**, to trigger the bot to cherry-pick this PR to your desired release version branch(es), you **must** add corresponding labels such as **needs-cherry-pick-1.1** and **needs-cherry-pick-1.0**.-->
-
 ### What is the related PR or file link(s)?
 
 <!--Give us some reference link(s) that might help quickly review and merge your PR, for example, a file link that supports why you changed the document.-->
@@ -28,5 +30,5 @@ If you are not sure of the options, leave it as it is. -->
 
 - [ ] Delete files
 - [ ] Change aliases
-- [ ] Have version specific changes <!-- If yes, please add the label "version-specific-changes-required"-->
-- [ ] Might cause conflicts
+- [ ] Need modification after applied to another branch <!-- If yes, please comment "/label version-specific-changes-required" below to trigger the bot to add the label.-->
+- [ ] Might cause conflicts after applied to another branch