Update icon guidelines (#11476)

### WHY are these changes introduced?

Let's contributors know that there's now an optimize script included to
help CI pass on added icons

polaris-icons/CONTRIBUTING.md

@@ -6,6 +6,7 @@ This repository is the place to contribute changes to icons and <https://polaris
 - [Icon sets and design guidelines](#icon-sets-and-design-guidelines)
 - [Proposing new icons](#proposing-new-icons)
 - [Proposing updates to existing icons](#proposing-updates-to-existing-icons)
+- [Adding new icons](#adding-new-icons)
 - [Deprecation guidelines](#deprecation-guidelines)
   - [Deleting an icon](#deleting-an-icon)
   - [Renaming an icon](#renaming-an-icon)
@@ -25,3 +26,10 @@ If you can’t find the icons you need, you can [propose new icons](https://gith
 ## Proposing updates to existing icons
 
 If you notice existing icons that are out-of-date or need improvements, you can [propose an update](https://github.com/Shopify/polaris/issues/new?assignees=&labels=Icon&template=UPDATE_ICON.yml&title=%5BIcon%5D%3A+Update+icon+%3Cicon-name%3E) to these icons.
+
+## Adding new icons
+
+1. Export your icon in SVG format from your design software.
+1. Copy and paste or move the SVG source into a new file: `polaris-icons/icons/{YourIconName}.svg`. (Make sure to suffix your icon name with `Icon` i.e. `BookOpenIcon.svg`).
+1. Add an accompanying YAML file: `polaris-icons/icons/{YourIconName}.yml`. The YAML file requires the following fields: `name, description, keywords, authors, date_added, date_modified`. See sibling files for reference.
+1. Optimize your icon by running `yarn optimize` from within the `polaris-icons` directory.

polaris-icons/icons/DatabaseConnectIcon.yml

@@ -12,5 +12,5 @@ keywords:
 authors:
   - Russell Baylis
   - Joe Thomas
-date_added: 2022-09-06
-date_modified: 2023-06-23
+date_added: 2024-01-23
+date_modified: 2024-01-23