Adds Marketplace link to README

[Stanzilla]

Thank you for taking the time to contribute to TypeScript + Webpack Problem Matchers! ♥

🚨 IMPORTANT 🚨

• Please create an issue before creating a Pull Request
• Please ensure to use the master branch as the base for all PRs
• Please use the following Git commit message style
  • Use the future tense ("Adds feature" not "Added feature")
  • Use a "Fixes #xxx -" or "Closes #xxx -" prefix to auto-close the issue that your PR addresses
  • Limit the first line to 72 characters or less
  • Reference issues and pull requests liberally after the first line

Description

Checklist

• [x] I have followed the guidelines in the Contributing document
• [x] My changes are based off of the master branch
• [x] My changes follow the coding style of this project
• [x] My changes build without any errors or warnings
• [x] My changes have been formatted and linted
• [x] My changes include any required corresponding changes to the documentation
• [x] My changes have been rebased and squashed to the minimal number (typically 1) of relevant commits
• [x] My changes have a descriptive commit message with a short title, including a Fixes $XXX - or Closes #XXX - prefix to auto-close the issue that your PR addresses

[SethFalco]

(Also has conflicts btw!)