* Update CONTRIBUTING.md - fixing a link, wording
Excited to see this unframework framework as a long time (since 1998 coder) HTML appreciator.
I wanted to learn about extensions but noticed a broken link that others might stumble upon.
Consider rephrasing a question to invite a person to contribute their issue. (this can be ignored if you like).
* Update CONTRIBUTING.md
Removed change to grammar/style of writing back to original.
Providing a more updated reference to extensions with greater detail.
* Update contribution guidelines
Break the PRs section into technical requirements and guidelines for how
we reviews. Add new info about how to craft PRs that are easy for us to
review.
* Fix typo
* Add note about extensions
* New extension guidelines