Update CONTRIBUTING.md

### Contributing code

If you have improvements to Milvus, send us your pull requests! For those just getting started, GitHub has a [how-to](https://help.github.com/en/articles/about-pull-requests).

If you have improvements to Milvus, send us your pull requests! For those just getting started, see [GitHub workflow](#github-workflow).

The Milvus team members will review your pull requests, and once it is accepted, it will be given a `ready to merge` label. This means we are working on submitting your pull request to the internal repository. After the change has been submitted internally, your pull request will be merged automatically on GitHub.

### GitHub workflow

Please create a new branch from an up-to-date master on your fork.

1. Fork the repository on GitHub.

2. Clone your fork to your local machine with `git clone git@github.com:<yourname>/milvus-io/milvus.git`.

3. Create a branch with `git checkout -b my-topic-branch`.

4. Make your changes, commit, then push to to GitHub with `git push --set-upstream origin my-topic-branch`.

5. Visit GitHub and make your pull request.

If you have an existing local repository, please update it before you start, to minimize the chance of merge conflicts.

```shell

git remote add upstream git@github.com:milvus-io/milvus.git

git checkout master

git pull upstream master

git checkout -b my-topic-branch

```

### General guidelines

### General guidelines

Before sending your pull requests for review, make sure your changes are consistent with the guidelines and follow the Milvus coding style.