Update CONTRIBUTING.md

[Thomas-Boi]

Contain:

• Remove references about contributors having to optimize the SVGs manually
• Update the requesting icon section

[Panquesito7]

👍 Good work, Thomas.

[Thomas-Boi]

IMPORTANT

The latest commit also fixes #693, which renames the vueStorefront folder to vuestorefront. Git Bash somehow doesn't register the name change as a commitable change => I had to batch it in with other changes. Let's see if it works. If it doesn't, I'll figure out another way

[amacado]

@Thomas-Boi i assume the vuestorefront fix was not successful. I approve this request anyway to update the CONTRIBUTING.md.