[web] [docs] add documentation to viewing icons in icomoon

Summary:
Add a comment in the icon component about how to view the icons in icomoon.

This is documentation. per the rules (https://www.notion.so/commapp/Diff-Review-Rules-addfc3a41b324c7d8ba636dd9dcefeba) adding ashoat.

Test Plan: go through the steps and make sure they make sense to you and anyone else on the team.

Reviewers: atul, palys-swm, ashoat

Reviewed By: ashoat

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3398