Added Polygon (Similar to Mongo but more Memory Efficient)

[JewishLewish]

Please check if what you want to add to awesome-go-storage list meets quality standards before sending pull request. Thanks!

Please provide package links to:

• github.com repo: https://github.com/JewishLewish/PolygonDB
• godoc.org: n/a (this is a database)
• goreportcard.com: github.com/JewishLewish/PolygonDB
• coverage service link (gocover, coveralls etc.): (again n/a)

Note: that new categories can be added only when there are 3 packages or more.

Make sure that you've checked the boxes below before you submit PR:

• I have added my package in alphabetical order
• I know that this package was not listed before
• I have added godoc link to the repo and to my pull request
• I have added coverage service link to the repo and to my pull request
• I have added goreportcard link to the repo and to my pull request
• I have read Contribution guidelines and Quality standard.

Thanks for your PR, you're awesome! 👍

[yufeifly]

您好，您的邮件已收到，谢谢。——熊宇飞