Update readme

[crisner]

Fixes #342 (<=== Add issue number here)

Make sure these boxes are checked before your pull request (PR) is ready to be reviewed and merged. Thanks!

• PR is descriptively titled 📑 and links the original issue above 🔗
• tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR
• code is in uniquely-named feature branch and has no merge conflicts 📁
• screenshots/GIFs are attached 📎 in case of UI updation
• ask @publiclab/reviewers for help, in a comment below

We're happy to help you get this ready -- don't be afraid to ask for help, and don't be discouraged if your tests fail at first!

If tests do fail, click on the red X to learn why by reading the logs.

Please be sure you've reviewed our contribution guidelines at https://publiclab.org/contributing-to-public-lab-software

Thanks!

[crisner]

• Updated documentation for fractracker mobile and for embed control
• Updated demo link
• Added documentation for layers browser menu

[sagarpreet-chadha]

Super!!!