Add iscream package sticker

[jamespeapen]

Thank you for contributing a new sticker to the BiocStickers repository!

Before submitting your pull request, please make sure you have completed the
following steps:

• I have read the
  CONTRIBUTING.md
  file.
• I have updated the README.md file in the main repository folder to
  include the new sticker.
• I have created a README.md file in the new sticker's folder.
• The README.md file in the new sticker's folder includes the following
  metadata:
  • Sticker / Package / Event name
  • Designer name and maintainer name (if different)
  • License information (otherwise, CC0 1.0 Universal Public Domain
    License applies by default)
  • Preview image of the sticker (with ~200px height)
  • Brief description of the sticker (optional)

Checklist

• The sticker is directly related to a Bioconductor package or project
• The sticker is in PNG, PDF, or SVG format
• The PNG sticker height is 2 inches (5 cm) and resolution is at least 300
  dpi
• The sticker is in a new folder with the same name as the sticker
• The sticker has a transparent background
• The sticker is licensed with a CC-BY-type license (otherwise, CC0 1.0
  Universal Public Domain License applies by default)