Move icon docs from the pattern library to the contribution guidelines
These guidelines are currently in the “Getting Started: Icons” page of the pattern library. These should be moved to a markdown file, like we've done for symbol-specific guidelines: https://gitlab.com/gitlab-org/gitlab-design/blob/master/doc/pattern-library.md#symbols
This is documentation specific to building the icons, not about using them. Usage guidelines are tackled by design.gitlab.com!60 (merged) and included in the Design System.
Other information, originally included in design.gitlab.com!60 (merged):
Follow these guidelines when developing icons: - Icons should be simple and use as few points as possible. The default bounding box size of icons is **16x16px**. Inside of that bounding box, some icons are larger, others are smaller, depending on their design. - Todo: Add do/don't examples - Use whole pixels to create icons in order to align properly to a pixel grid. - Todo: Add do/don't examples - Use rounded strokes. - Todo: Add do/don't examples - Simplify icons for clarity and legibility. - Todo: Add do/don't examples - Make the icons front-face instead of different dimensions. - Todo: Add do/don't examples … * Border thickness: 2px * Border radius: 3px Exporting icons as SVG ---------------------- When exporting SVGs, be sure to follow the following guidelines: 1. Convert all strokes to outlines. 2. Use pathfinder tools to combine overlapping paths and create compound paths. 3. SVGs that are limited to one color should be exported without a fill color so the color can be set using CSS. 4. Ensure that exported SVGs have been run through an [SVG cleaner](https://github.com/RazrFalcon/SVGCleaner) to remove unused elements and attributes. You can open your svg in a text editor to ensure that it is clean. Incorrect files will look like this: Todo: Add SVG code example Correct file will look like this: Todo: Add SVG code example