Add implementation guide

source/implementation.liquid

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+---
+layout: has_no_sidebars
+title: Implementation Guide
+introduction: >
+  Sass has a thriving community of implementations, with more being produced all
+  the time. The core team loves to see new implementations thrive and mature,
+  and they want to help out in any way they can.
+---
+
+<ul class="list-tiled">
+<li>
+{% markdown %}
+## Resources
+
+* [`sass-spec`](https://github.com/sass/sass-spec) is a suite of
+  implementation-agnostic test cases for verifying that a Sass
+  implementation behaves correctly. It's the best way to track your
+  implementation's compatibility with the Sass reference implementation.
+
+* [How `@extend` Works](https://gist.github.com/nex3/7609394) is a
+  fairly comprehensive run-down of the algorithm used by Sass's
+  trickiest feature. Natalie still says that the implementation of
+  `@extend` is the hardest code she's ever had to write, but luckily you
+  don't have to figure it out from scratch.
+
+* **Reach out!** If you're working on a new implementation, we want to
+  hear about it. Send an email to [Natalie](mailto:nex342@gmail.com) and
+  [Chris](mailto:chris@eppsteins.net), tell us about the cool work
+  you're doing, and ask about any corners of the language that don't
+  quite make sense.
+{% endmarkdown %}
+</li>
+
+<li>
+{% markdown %}
+## Requirements
+
+We whole-heartedly love new implementations of Sass, but we do have a
+few restrictions that we ask those implementations to follow in order to
+call themselves "Sass", "Sass implementations", or the like. Sass is a
+community as much as it is a language, and it's important that all
+implementations are willing to work for the good of the community.
+
+First, we ask that every implementation adopt the [Sass community
+guidelines](/community-guidelines) for their own implementation-specific
+communities. Much of what makes the Sass community strong is a culture
+of kindness and respect, and having clear and explicit guidelines helps
+produce that culture.
+
+Second, we ask that implementations not extend the language without
+agreement from the other major implementations and from the language
+designers, Natalie and Chris. The only reason a Sass community exists at
+all is because the language enables styles and frameworks to be shared
+among designers, and it's crucial for sharing that Sass code that works
+for one implementation works the same for all of them. In addition, it's
+important that there be a unified vision for the language design.
+{% endmarkdown %}
+</li>
+
+<li>
+{% markdown %}
+## Making Language Changes
+
+Sass can still evolve as a language, of course. We have [a process][] for
+proposing and iterating on new language features that anyone can
+participate in. Language changes are discussed collaboratively, with
+particular weight given to the maintainers of mature Sass implementations.
+Attempts will be made to reach consensus with all stakeholders. However,
+this may be impossible in some circumstances, and the ultimate say lies
+with the lead designer of Sass, Natalie.
+
+[a process]: https://github.com/sass/sass/blob/main/CONTRIBUTING.md
+{% endmarkdown %}
+</li>
+</ul>