Add link to opentelemetry.io under About section

[chalin]

Under the About section of this repo (which you can access from the main repo page -- click the gear icon next to the About title):

• Under Website, add https://opentelemetry.io

We're looking to do this uniformly for all OTel org repos.

For example:

[arminru]

Thank you for your suggestion @chalin, I added it!