Add Sermant logos

[lilai23]

Pre-submission checklist:

If you're adding a new project to the repo, please confirm each of these actions has been completed:

• Artwork is supplied in both PNG and SVG formats.
• SVGs are real vector-based files. Test and optimize them at https://autocrop.cncf.io before submitting.
• Artwork includes 3 layouts: horizontal (landscape), stacked, and icon.
• At least 3 versions of the artwork are provided: color, black, and white.
• Files follow the existing repo folder and naming conventions.
• The correct examples/file.md is updated with a section for the logos added, in A-Z order.
• A link to the new logo section is added to the main README.md, in A-Z order.

[lilai23]

Hi, this is a pull request about uploading logos for sandbox project Sermant. Please review it. Thanks！
cncf/sandbox#300

[lukaszgryglicki]

/lgtm

[krook]

Thanks @lilai23!