Update documentation about adding new icons

[guastallaigor]

Is your feature request related to a problem? Please describe.
We are getting PRs about adding new icons, and we are not accepting those, because of the bundle size.

References:

• feat(icons.scss): add 2 china apps icons #410 (comment)
• CodePen icon #310 (comment)

Most contributors creates new PRs without opening issues first, which is unfortunate.

Describe the solution you'd like
Add a full section inside the README.md and / or CONTRIBUTION.md that specifically says: "We are not accepting PRs that adds new icons from the time being", or something like that.

Describe alternatives you've considered
We could also create a checklist in the PRs, similar to the other repos, that checks everything we are looking for in a PR, and one of those topics is something about this.

Or just inform the contributors on Gitter/Discord and only on open issues / PRs.

Additional context
Just see the PRs that are open and some of them closed regarding this issue.