README: add link to demos

[dolmen]

From the Hacker News post, those demos would be helpful to show in the README:

• https://segfault87.github.io/ldraw-rs-preview/#models/car.ldr
• https://segfault87.github.io/ldraw-rs-preview/#models/6973.ldr

[segfault87]

Sounds good. I'm going to completely rewrite README and will attach link to demos.

[segfault87]

Added link to demo pages.