Add Marketplace link to github page

[RedCMD]

Add https://marketplace.visualstudio.com/items?itemName=zokugun.explicit-folding under links (About)

[daiyam]

First, you haven't read the README issue #153
So, from now on, please follow that README and the CONTRIBUTING.md.

Back your problem, the extension is available on the VSCode Marketplace and OpenVSX. Both already have link to them in the badge.
Why would I prioritize the Marketplace and be my official page?