Make the governance policy for this repo clear (spotbugs#1856)

* docs: make the governance policy for this repo clear

* docs: reflect a review from ThrawnCA

https://github.com/spotbugs/spotbugs/pull/1856/files#r765329718

* docs: update GOVERNANCE.md

* docs: correct sentences to tell meaning without making readers confused

.github/CONTRIBUTING.md

@@ -14,3 +14,8 @@ Just three steps to follow:
 ## Before you propose new rules
 
 Please consider to follow the same points with ***Before reporting problem in detectors***.
+
+## Before you submit a pull request
+
+1. Run `./gradlew spotlessApply build smoketest` in your local to verify your change.
+2. Make sure you updated the `CHANGELOG.md` accordingly. Detailed requirements are explained at the beginning of the changelog.

GOVERNANCE.md

@@ -0,0 +1,29 @@
+# Governance Policy
+
+This document provides the governance policy for the SpotBugs Project.
+
+## SpotBugs Core Team
+
+The SpotBugs Core Team maintains the spotbugs/spotbugs GitHub repository.
+
+The GitHub team for SpotBugs Team is `@spotbugs/core-devs`. Members of the SpotBugs Core Team have write access to the repository.
+
+## How we propose changes
+
+Both SpotBugs Core Team and SpotBugs user can propose changes to the SpotBugs project via GitHub pull requests. Refer to `.github/CONTRIBUTING.md` to know more detailed requirements for your proposal.
+
+Once a pull request gets two approvals from members of SpotBugs Core Team, pull request can be merged and shipped in the next release.
+
+If pull request cannot get enough approvals during 30 days, it can be labeled as stale pull request. Stale pull requests can be closed after another 30 days.
+
+## Issue Management Policy
+
+Both SpotBugs Core Team and SpotBugs users can create issues at GitHub Issues. Refer to `.github/CONTRIBUTING.md` to get more detailed requirements for your proposal.
+
+If issue is not updated for 30 days from when it was labeled as 'need info', SpotBugs Core Team can close it.
+
+## Nomination to the SpotBugs Core Team
+
+Members of SpotBugs Core Team can nominate new members. The nomination process is held at [GitHub Team Discussions](https://docs.github.com/en/organizations/collaborating-with-your-team/about-team-discussions).
+
+We set no clear condition for nomination, but usually nominate from active contributors.

README.md

@@ -33,4 +33,4 @@ SpotBugs can be used standalone and through several integrations, including:
 * [IntelliJ IDEA](https://github.com/JetBrains/spotbugs-intellij-plugin)
 
 # Questions?
-You can contact us using [our general purpose mailing list](https://github.com/spotbugs/discuss/issues?q=).
+You can contact us using [GitHub Discussions](https://github.com/spotbugs/spotbugs/discussions).