To contribute your own icon pack, please submit a pull request. Put your icons into a new folder and make sure to include a file named metadata.json in it.
The structure of metadata.json is as follows:

{
  "author": {
    "name": "<Your name>",
    "github": "<Your GitHub Name>"
  },
  "name": "<Your icon pack name>",
  "version": "<Your icon pack version>",
  "format": 1,
  "ondark": <true if your icons need to be presented on a dark background, otherwise false>
}

Warning

Please make sure all of your files are actually valid ICO files. An easy way to convert from PNG to ICO is to use ImageMagick and use the following command:

magick mogrify -format ico *.png

To verify, please use the Validate-Icos.ps1 script. It will point out every ico file with an invalid format.

Also, please note that all uploaded icons will fall under the CC BY-NC-SA license.