Add README

Author: jdesrosiers

README.md

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 # JSON Schema (lite)
+
+**_NOT FOR PRODUCTION USE_**
+
+This is a minimal implementation of JSON Schema 2020-12 with a couple of the
+more complex features left out. This implementation exists solely to facilitate
+the qualification task for the JSON Schema GSoC project [Better JSON Schema
+Errors](https://github.com/json-schema-org/community/issues/870) project. Do not
+use it in production. It will not be maintained or supported.
+
+## Qualification Task
+
+There's no better way to get familiar with the JSON Schema output format than to
+implement it yourself. This implementation currently implements the Flag output
+format. Your task is to update it to support either the Basic or Detailed output
+formats. You must include `valid`, `absoluteKeywordLocation`, and
+`instanceLocation`. Because we aren't implementing the Verbose output format,
+you don't need to support `annotations`/`annotation`. You don't need to support
+`error` because we're going to ignore messages from the implementation anyway.
+`keywordLocation` is optional because I can't see any reason we'd use it. You
+must include tests to show that your implementation works including coverage for
+all implemented keywords.
+
+To submit your qualification task, use `npm pack` and DM it to me. I will
+provide one and only one review for each candidate, so make sure you're ready
+when you submit. You can't fail the qualification task. As long as you submit
+something, I'll consider your application. However, I will strongly take into
+consideration the quality of your submission when evaluating applications.
+
+This project uses ESLint. I encourage you to install an ESLint plugin in your
+editor to get feedback in real time. ESLint is also used for code style checks,
+but it doesn't check everything, so make an effort to match my code style when
+making changes.
+
+There are three scripts you should make sure that you are passing before
+submitting.
+
+- `npm test`
+- `npm run lint`
+- `npm run type-check`
+
+I strongly encourage you to turn off your AI coding assistants for this
+exercise. They tend to generate very low quality code. Code that works is just
+the beginning. It also needs to be maintainable. Sloppy code hides bugs, is hard
+to maintain, and slows down progress in the long term. This may be throw away
+code that we're never going to build on later, but remember that part of the
+goal of this task is to demonstrate your ability to write quality code for a
+project that will need to evolve and be maintained over the several next years.
+
+## About the Implementation
+
+Unsupported features and keywords
+- Embedded schemas
+- `$anchor`
+- `$dynamicRef`/`$dynamicAnchor`
+- `unevaluatedProperties`/`unevaluatedItems`
+- `format` assertion
+- Annotations
+- Custom dialect/vocabularies/keywords
+- Older dialects
+- Retrieving files from the file system or the web
+
+### API
+
+* `validate(schema: Json, instance: Json) => Output`
+* `registerSchema(schema: Json, uri: string) => void`
+
+The `Json` type represents any JavaScript value that is compatible with JSON.
+
+The `Output` type represents the Flag output format. It includes the `valid`
+property, but no `errors`.
+
+### Example Usage
+
+```javascript
+import { validate } from "../src/index.js";
+
+/**
+ * @import { Json } from "./src/jsonast.d.ts"
+ */
+
+/** @type Json */
+const schema = {
+  $ref: "#/$defs/a",
+  $defs: {
+    a: { type: "number" }
+  }
+};
+const instance = true;
+const output = validate(schema, instance);
+console.log("valid", output.valid);
+```