docs: wrote basic instructions for authoring a rule

Author: elijah-potter

packages/web/src/routes/docs/contributors/author_a_rule/+page.md

@@ -0,0 +1,28 @@
+---
+title: Author a Rule
+---
+
+[Harper's grammatical rules are many](../rules), but most are relatively easy to understand.
+Before we get into how to write a rule, it is important that we get some of the language cleared up.
+
+When we refer to a Harper rule, we are talking about [an implementation of the Linter trait](https://docs.rs/harper-core/latest/harper_core/linting/trait.Linter.html).
+As you can see, there is an enormous amount of flexibility in this trait and a wide variety of strategies for querying the provided document.
+
+## Patterns
+
+For new contributors, defining a pattern and [implementing the PatternLinter trait](https://docs.rs/harper-core/latest/harper_core/linting/trait.PatternLinter.html) is the easiest way to get started.
+If you like to learn by example, you can make a copy of the `ThatWhich` rule and modify it to look for something else.
+Here's the general playbook:
+
+- Start by forking the [Harper monorepo](https://github.com/Automattic/harper/fork) and create a new file under `harper-core/src/linting` called `my_rule.rs`.
+- Follow our [guide to get your environment setup.](./environment)
+- Copy in a template rule (like from `that_which.rs`).
+- Modify the constructor to create a pattern to look for the problematic text you have in mind.
+- Export your rule from the `linting module` ([which you can find here](https://github.com/Automattic/harper/blob/master/harper-core/src/linting/mod.rs))
+- Add your rule to the `LintGroup` [macro call](https://github.com/Automattic/harper/blob/master/harper-core/src/linting/lint_group.rs), which will aggregate its results with the other linters in Harper.
+- Open a PR.
+
+## Querying the Document Directly
+
+If you don't wish to use a Harper [Pattern](https://docs.rs/harper-core/latest/harper_core/patterns/trait.Pattern.html), you may query the `Document` directly.
+Make sure you read the [available methods](https://docs.rs/harper-core/latest/harper_core/struct.Document.html) available for `Document`s and for [TokenStrings](https://docs.rs/harper-core/latest/harper_core/struct.Document.html#impl-TokenStringExt-for-Document).

packages/web/vite.config.ts

@@ -112,6 +112,10 @@ export default defineConfig({
 								{
 									title: 'Dictionary',
 									to: '/docs/contributors/dictionary'
+								},
+								{
+									title: 'Author a Rule',
+									to: '/docs/contributors/author_a_rule'
 								}
 							]
 						},