Document new doc= feature for validator UI. (ampproject#20610)

* docs

* change link

validator/webui/README.md

@@ -32,3 +32,21 @@ Then visit your own instance at http://127.0.0.1:8765/.
 If you'd like to run exactly the code that is running at
 [validator.ampproject.org](https://validator.ampproject.org/), that's an
 Appengine app - please refer to the instructions in serve.go.
+
+## Passing in documents from URL
+
+This tool will also accept a document passed in via the URL as part of the URL hash. It expects the format `#doc=<ENCODED_DOCUMENT>`. This allows users to construct an link that will auto-populate the tool with the incoming document.
+
+To use this feature you must first base64 encode the HTML string. Due to unicode problems with the native `btoa()` function, this tool expects the string to be encoded using the following [solution from mdn](https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/btoa#Unicode_strings):
+```js
+// ucs-2 string to base64 encoded ascii
+function utoa(str) {
+    return window.btoa(unescape(encodeURIComponent(str)));
+}
+```
+
+By default this tool will assume you want to validate an `AMPHTML` document. If you would like to validate another format you can add `#htmlFormat={'AMP4ADS', 'AMP4EMAIL', 'ACTIONS'}`.
+
+If you wish to use both the `doc=` and `htmlFormat=` together make sure to include an `&` between the key-value pairs.
+
+Putting it all together allows you to create links like [this example.](https://validator.ampproject.org/#htmlFormat=AMP4ADS&doc=PCFkb2N0eXBlIGh0bWw%2BCjxodG1sIOKaoTRhZHM%2BCjxoZWFkPgogIDxtZXRhIGNoYXJzZXQ9InV0Zi04Ij4KICA8bWV0YSBuYW1lPSJ2aWV3cG9ydCIgY29udGVudD0id2lkdGg9ZGV2aWNlLXdpZHRoLG1pbmltdW0tc2NhbGU9MSI%2BCiAgPHN0eWxlIGFtcDRhZHMtYm9pbGVycGxhdGU%2BYm9keXt2aXNpYmlsaXR5OmhpZGRlbn08L3N0eWxlPgogIDxzY3JpcHQgYXN5bmMgc3JjPSJodHRwczovL2Nkbi5hbXBwcm9qZWN0Lm9yZy9hbXA0YWRzLXYwLmpzIj48L3NjcmlwdD4KPC9oZWFkPgo8Ym9keT4KCTxhbXAtaW1nIHdpZHRoPSI1MDAiIGhlaWdodD0iNTAwIiBzcmM9Imh0dHA6Ly9wbGFjZWtpdHRlbi5jb20vNTAwLzUwMCI%2BPC9hbXAtaW1nPgogIAk8aDE%2BQ2F0cyBhcmUgY29vbC48L2gxPgo8L2JvZHk%2BCjwvaHRtbD4)