docs: updating CONTRIBUTING.md

Add codestyle guides about golangci-lint.
Add git commit message guide and examples.

Author: IronCore864

CONTRIBUTING.md

@@ -20,6 +20,43 @@ We aim to reply to issues within 24 hours.
 
 ## Style Guides
 
+### Linters
+
+We use `golangci-lint` ([official website](https://golangci-lint.run/), [GitHub](https://github.com/golangci/golangci-lint)) for linting, which is a Go linters aggregator. It's also integrated with the GitHub Actions workflows.
+
+- The list of linters enabled by default is documented [here](https://golangci-lint.run/usage/linters/).
+- It can be [integrated with IDE](https://golangci-lint.run/usage/integrations/)
+- You can [run it locally](https://golangci-lint.run/usage/quick-start/).
+
+Besides, we also use the [Go Report Card](https://goreportcard.com/report/github.com/merico-dev/stream). There is a badge like [![Go Report Card](https://goreportcard.com/badge/github.com/merico-dev/stream)](https://goreportcard.com/report/github.com/merico-dev/stream) in the main README.md.
+
 ### Git Commit Message
 
 We try our best to follow the [conventional commits](https://www.conventionalcommits.org/en/v1.0.0/#summary) guidelines.
+
+TL;DR: The commit message should be structured as follows:
+
+```
+<type>[optional scope]: <description>
+[optional body]
+[optional footer(s)]
+```
+
+where "type" can be:
+- `feat`: implements a feature
+- `fix`: patches a bug
+- `BREAKING CHANGE`: a breaking change. Or append `!` at the end of "feat" or "fix", like `feat!` and `fix!`
+- other types are allowed, for example: `build`, `chore`, `ci`, `docs`, `style`, `refactor`, `perf`, `test`
+
+Both "body" and "footer" are optional; BREAKING CHANGE can be addressed both in the title as well as in the footer. Some examples:
+
+- `feat: send an email to the customer when a product is shipped`
+- `feat!: send an email to the customer when a product is shipped`
+- `feat(api): send an email to the customer when a product is shipped`
+- `feat(api)!: send an email to the customer when a product is shipped`
+- `BREAKING CHANGE: send an email to the customer when a product is shipped`
+- ```
+  feat!: send an email to the customer when a product is shipped
+  A detailed description in the body.
+  BREAKING CHANGE: readdressing the breaking change in the footer.
+  ```