Add basic docs for adding WPT test suites

src/workerd/api/wpt/README.md

@@ -0,0 +1,45 @@
+## How to add WPT test suites to workerd
+
+[WPT groups its test suites into directories](https://github.com/web-platform-tests/wpt). To add a new test suite to workerd, simply create a file named `suite-test-config.js`. For example, `url/` tests are added in the `url-test-config.js` file.
+
+A test config file is expected to export an object named `config`. This object can be empty to run all tests and subtests.
+
+```js
+export const config = {};
+```
+
+A test export is generated for each JS file in the directory. WPT test files are sometimes located in subdirectories, but the test importer flattens the hierarchy within the test suite.
+
+Each test file can contain several subtests, created by invoking `test` or `promise_test` within the test file. They are named according to the message argument provided.
+
+## Options
+
+Each entry in the `config` object specifies the test file name, and the options to apply to those tests. For example, `urlsearchparams-sort.any.js` can be configured using:
+
+```js
+export const config = {
+    'urlsearchparams-sort.any.js': {
+        expectedFailures: ['Parse and sort: ffi&🌈', 'URL parse and sort: ffi&🌈'],
+    },
+};
+```
+
+
+The following options are currently supported:
+
+* `ignore: bool`: Don't import a test file that is irrelevant for workerd tests
+* `skippedTests: string[]`: A list of subtests that should not be executed. This should only be used for subtests that would crash workerd.
+* `expectedFailures: string[]`: A list of subtests that are expected to fail, either due to a bug in workerd or an intentional choice not to support a feature.
+
+## Implementation
+
+Once a test config file is detected, the `wpt_test` macro is invoked for the suite. A JS test file is created that invokes the test harness on each JS file within the suite.
+
+A WD test file is generated which links all of the necessary components:
+
+* Generated JS test file (e.g. `url-test.js`)
+* The test config file (e.g. `url-test-config.js`)
+* Test harness (`harness.js`)
+* Test files provided by WPT  (e.g. `url/urlsearchparams-sort.any.js`)
+* JSON resources provided by WPT (e.g. `url/resources/urltestdata.json`)
+