Dashicons, the WordPress admin icon font. For the official documentation, please refer to the WordPress Developer Resource.
For new icon requests, please post a request on the WordPress trac ticket pertaining to new Dashicons for the next version of WordPress. If you'd like to work an an icon request, check out our WIP Dashicons style guide.
You can join Dashicons discussion and get help on the official WordPress Slack chat. Just register and join the #design-dashicons channel.
For any bugs that appear within WordPress Core, please create a new ticket on WordPress Trac. Use the "Administration" component and the "ui" focus when creating the new ticket, and be sure to include "Dashicons" somewhere in the text of the ticket.
To build Dashicons, make sure you have Node JS installed. Then type npm run build on the commandline to generate the minified SVG files and sprite.
Once you're installed, to add an icon, save an SVG in the sources/svg folder, then run the npm run build command on the commandline.
If you want to add your icon to the Dashicons library, open a new Pull Request here with the new SVG icon, and wait a review.
If you're adding an icon for Gutenberg specifically, add it inside the sources/svg/gutenberg folder. It will be processed for everything but the font files.
Dashicons is licensed under GPLv2, or any later version with font exception.