docs: add README to cli/js/web/ (#4578)

cli/js/web/README.md

@@ -6,3 +6,45 @@ Please note, that some of implementations might not be completely aligned with
 specification.
 
 Some of the Web APIs are using ops under the hood, eg. `console`, `performance`.
+
+## Implemented Web APIs
+
+- [Blob](https://developer.mozilla.org/en-US/docs/Web/API/Blob): for
+  representing opaque binary data
+- [Console](https://developer.mozilla.org/en-US/docs/Web/API/Console): for
+  logging purposes
+- [CustomEvent](https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent),
+  [EventTarget](https://developer.mozilla.org/en-US/docs/Web/API/EventTarget)
+  and
+  [EventListener](https://developer.mozilla.org/en-US/docs/Web/API/EventListener):
+  to work with DOM events
+  - **Implementation notes:** There is no DOM hierarchy in Deno, so there is no
+    tree for Events to bubble/capture through.
+- [URL](https://developer.mozilla.org/en-US/docs/Web/API/URL) and
+  [URLSearchParams](https://developer.mozilla.org/en-US/docs/Web/API/URLSearchParams):
+  to construct and parse URLSs
+- [fetch](https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch),
+  [Request](https://developer.mozilla.org/en-US/docs/Web/API/Request),
+  [Response](https://developer.mozilla.org/en-US/docs/Web/API/Response),
+  [Body](https://developer.mozilla.org/en-US/docs/Web/API/Body) and
+  [Headers](https://developer.mozilla.org/en-US/docs/Web/API/Headers): modern
+  Promise-based HTTP Request API
+- [FormData](https://developer.mozilla.org/en-US/docs/Web/API/FormData): access
+  to a `multipart/form-data` serialization
+- [Location](https://developer.mozilla.org/en-US/docs/Web/API/Location): parsing
+  the current script's URL
+  - **Implementation notes:** the `globalThis.location` object cannot be
+    manipulated using `assign()`, `reload()` and `replace()` methods. They are
+    not implemented.
+- [Performance](https://developer.mozilla.org/en-US/docs/Web/API/Performance):
+  retrieving current time with a high precision
+- [setTimeout](https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/setTimeout),
+  [setInterval](https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/setInterval),
+  [clearTimeout](https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/clearTimeout):
+  scheduling callbacks in future and
+  [clearInterval](https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/clearInterval)
+- [Worker](https://developer.mozilla.org/en-US/docs/Web/API/Worker): executing
+  additional code in a separate thread
+  - **Implementation notes:** Blob URLs are not supported, object ownership
+    cannot be transferred, posted data is serialized to JSON instead of
+    [structured cloning](https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm).